

<!--DFS XBRL Instance Document - http://www.dfsco.com/financial-services/ - Version R3.0 - on 05-March-2019 [10:59:59] {AM}-->
<linkbase xmlns="http://www.xbrl.org/2003/linkbase" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xlink="http://www.w3.org/1999/xlink" xsi:schemaLocation="http://www.xbrl.org/2003/linkbase http://www.xbrl.org/2003/xbrl-linkbase-2003-12-31.xsd" xmlns:xbrldt="http://xbrl.org/2005/xbrldt">
<roleRef xlink:type="simple" xlink:href="galt-20181231.xsd#DocumentAndEntityInformation" roleURI="http://www.galectintherapeutics.com/role/DocumentAndEntityInformation" />
<roleRef xlink:type="simple" xlink:href="galt-20181231.xsd#ConsolidatedBalanceSheets" roleURI="http://www.galectintherapeutics.com/role/ConsolidatedBalanceSheets" />
<roleRef xlink:type="simple" xlink:href="galt-20181231.xsd#ConsolidatedBalanceSheetsParenthetical" roleURI="http://www.galectintherapeutics.com/role/ConsolidatedBalanceSheetsParenthetical" />
<roleRef xlink:type="simple" xlink:href="galt-20181231.xsd#ConsolidatedStatementsOfOperations" roleURI="http://www.galectintherapeutics.com/role/ConsolidatedStatementsOfOperations" />
<roleRef xlink:type="simple" xlink:href="galt-20181231.xsd#ConsolidatedStatementsOfChangesInRedeemableConvertiblePreferredStockAndStockholdersEquityDeficit" roleURI="http://www.galectintherapeutics.com/role/ConsolidatedStatementsOfChangesInRedeemableConvertiblePreferredStockAndStockholdersEquityDeficit" />
<roleRef xlink:type="simple" xlink:href="galt-20181231.xsd#ConsolidatedStatementsOfCashFlows" roleURI="http://www.galectintherapeutics.com/role/ConsolidatedStatementsOfCashFlows" />
<roleRef xlink:type="simple" xlink:href="galt-20181231.xsd#NatureOfBusinessAndBasisOfPresentation" roleURI="http://www.galectintherapeutics.com/role/NatureOfBusinessAndBasisOfPresentation" />
<roleRef xlink:type="simple" xlink:href="galt-20181231.xsd#SummaryOfSignificantAccountingPolicies" roleURI="http://www.galectintherapeutics.com/role/SummaryOfSignificantAccountingPolicies" />
<roleRef xlink:type="simple" xlink:href="galt-20181231.xsd#PropertyAndEquipment" roleURI="http://www.galectintherapeutics.com/role/PropertyAndEquipment" />
<roleRef xlink:type="simple" xlink:href="galt-20181231.xsd#AccruedExpenses" roleURI="http://www.galectintherapeutics.com/role/AccruedExpenses" />
<roleRef xlink:type="simple" xlink:href="galt-20181231.xsd#StockholdersEquity" roleURI="http://www.galectintherapeutics.com/role/StockholdersEquity" />
<roleRef xlink:type="simple" xlink:href="galt-20181231.xsd#Warrants" roleURI="http://www.galectintherapeutics.com/role/Warrants" />
<roleRef xlink:type="simple" xlink:href="galt-20181231.xsd#StockBasedCompensation" roleURI="http://www.galectintherapeutics.com/role/StockBasedCompensation" />
<roleRef xlink:type="simple" xlink:href="galt-20181231.xsd#LineOfCredit" roleURI="http://www.galectintherapeutics.com/role/LineOfCredit" />
<roleRef xlink:type="simple" xlink:href="galt-20181231.xsd#LossPerShare" roleURI="http://www.galectintherapeutics.com/role/LossPerShare" />
<roleRef xlink:type="simple" xlink:href="galt-20181231.xsd#CommitmentsAndContingencies" roleURI="http://www.galectintherapeutics.com/role/CommitmentsAndContingencies" />
<roleRef xlink:type="simple" xlink:href="galt-20181231.xsd#GalectinSciencesLlc" roleURI="http://www.galectintherapeutics.com/role/GalectinSciencesLlc" />
<roleRef xlink:type="simple" xlink:href="galt-20181231.xsd#IncomeTaxes" roleURI="http://www.galectintherapeutics.com/role/IncomeTaxes" />
<roleRef xlink:type="simple" xlink:href="galt-20181231.xsd#SubsequentEvents" roleURI="http://www.galectintherapeutics.com/role/SubsequentEvents" />
<roleRef xlink:type="simple" xlink:href="galt-20181231.xsd#SummaryOfSignificantAccountingPoliciesPolicies" roleURI="http://www.galectintherapeutics.com/role/SummaryOfSignificantAccountingPoliciesPolicies" />
<roleRef xlink:type="simple" xlink:href="galt-20181231.xsd#PropertyAndEquipmentTables" roleURI="http://www.galectintherapeutics.com/role/PropertyAndEquipmentTables" />
<roleRef xlink:type="simple" xlink:href="galt-20181231.xsd#AccruedExpensesTables" roleURI="http://www.galectintherapeutics.com/role/AccruedExpensesTables" />
<roleRef xlink:type="simple" xlink:href="galt-20181231.xsd#StockholdersEquityTables" roleURI="http://www.galectintherapeutics.com/role/StockholdersEquityTables" />
<roleRef xlink:type="simple" xlink:href="galt-20181231.xsd#WarrantsTables" roleURI="http://www.galectintherapeutics.com/role/WarrantsTables" />
<roleRef xlink:type="simple" xlink:href="galt-20181231.xsd#StockBasedCompensationTables" roleURI="http://www.galectintherapeutics.com/role/StockBasedCompensationTables" />
<roleRef xlink:type="simple" xlink:href="galt-20181231.xsd#LossPerShareTables" roleURI="http://www.galectintherapeutics.com/role/LossPerShareTables" />
<roleRef xlink:type="simple" xlink:href="galt-20181231.xsd#CommitmentsAndContingenciesTables" roleURI="http://www.galectintherapeutics.com/role/CommitmentsAndContingenciesTables" />
<roleRef xlink:type="simple" xlink:href="galt-20181231.xsd#IncomeTaxesTables" roleURI="http://www.galectintherapeutics.com/role/IncomeTaxesTables" />
<roleRef xlink:type="simple" xlink:href="galt-20181231.xsd#NatureOfBusinessAndBasisOfPresentationAdditionalInformationDetail" roleURI="http://www.galectintherapeutics.com/role/NatureOfBusinessAndBasisOfPresentationAdditionalInformationDetail" />
<roleRef xlink:type="simple" xlink:href="galt-20181231.xsd#SummaryOfSignificantAccountingPoliciesAdditionalInformationDetail" roleURI="http://www.galectintherapeutics.com/role/SummaryOfSignificantAccountingPoliciesAdditionalInformationDetail" />
<roleRef xlink:type="simple" xlink:href="galt-20181231.xsd#PropertyAndEquipmentDetail" roleURI="http://www.galectintherapeutics.com/role/PropertyAndEquipmentDetail" />
<roleRef xlink:type="simple" xlink:href="galt-20181231.xsd#PropertyAndEquipmentAdditionalInformationDetail" roleURI="http://www.galectintherapeutics.com/role/PropertyAndEquipmentAdditionalInformationDetail" />
<roleRef xlink:type="simple" xlink:href="galt-20181231.xsd#AccruedExpensesDetail" roleURI="http://www.galectintherapeutics.com/role/AccruedExpensesDetail" />
<roleRef xlink:type="simple" xlink:href="galt-20181231.xsd#StockholdersEquityAdditionalInformationDetail" roleURI="http://www.galectintherapeutics.com/role/StockholdersEquityAdditionalInformationDetail" />
<roleRef xlink:type="simple" xlink:href="galt-20181231.xsd#ReturnOnInvestmentDetail" roleURI="http://www.galectintherapeutics.com/role/ReturnOnInvestmentDetail" />
<roleRef xlink:type="simple" xlink:href="galt-20181231.xsd#SummaryOfWarrantActivityDetail" roleURI="http://www.galectintherapeutics.com/role/SummaryOfWarrantActivityDetail" />
<roleRef xlink:type="simple" xlink:href="galt-20181231.xsd#SummaryOfInformationWithRegardToOutstandingWarrantsIssuedInConnectionWithEquityAndDebtFinancingAndConsultantsDetail" roleURI="http://www.galectintherapeutics.com/role/SummaryOfInformationWithRegardToOutstandingWarrantsIssuedInConnectionWithEquityAndDebtFinancingAndConsultantsDetail" />
<roleRef xlink:type="simple" xlink:href="galt-20181231.xsd#SummaryOfInformationWithRegardToOutstandingWarrantsIssuedInConnectionWithEquityAndDebtFinancingAndConsultantsParentheticalDetail" roleURI="http://www.galectintherapeutics.com/role/SummaryOfInformationWithRegardToOutstandingWarrantsIssuedInConnectionWithEquityAndDebtFinancingAndConsultantsParentheticalDetail" />
<roleRef xlink:type="simple" xlink:href="galt-20181231.xsd#WarrantsAdditionalInformationDetail" roleURI="http://www.galectintherapeutics.com/role/WarrantsAdditionalInformationDetail" />
<roleRef xlink:type="simple" xlink:href="galt-20181231.xsd#StockBasedCompensationAdditionalInformationDetail" roleURI="http://www.galectintherapeutics.com/role/StockBasedCompensationAdditionalInformationDetail" />
<roleRef xlink:type="simple" xlink:href="galt-20181231.xsd#StockBasedCompensationExpenseRelatedToCommonStockOptionsRestrictedCommonStockAndCommonStockWarrantsDetail" roleURI="http://www.galectintherapeutics.com/role/StockBasedCompensationExpenseRelatedToCommonStockOptionsRestrictedCommonStockAndCommonStockWarrantsDetail" />
<roleRef xlink:type="simple" xlink:href="galt-20181231.xsd#WeightedAverageAssumptionsUsedToDetermineFairValueOfOptionsGrantedDetail" roleURI="http://www.galectintherapeutics.com/role/WeightedAverageAssumptionsUsedToDetermineFairValueOfOptionsGrantedDetail" />
<roleRef xlink:type="simple" xlink:href="galt-20181231.xsd#SummaryOfStockOptionActivityDetail" roleURI="http://www.galectintherapeutics.com/role/SummaryOfStockOptionActivityDetail" />
<roleRef xlink:type="simple" xlink:href="galt-20181231.xsd#SummaryOfAdditionalInformationRegardingOutstandingAndExercisableOptionsUnderStockBasedCompensationPlansDetail" roleURI="http://www.galectintherapeutics.com/role/SummaryOfAdditionalInformationRegardingOutstandingAndExercisableOptionsUnderStockBasedCompensationPlansDetail" />
<roleRef xlink:type="simple" xlink:href="galt-20181231.xsd#LineOfCreditAdditionalInformationDetail" roleURI="http://www.galectintherapeutics.com/role/LineOfCreditAdditionalInformationDetail" />
<roleRef xlink:type="simple" xlink:href="galt-20181231.xsd#ComputationOfDilutedNetLossPerShareDetail" roleURI="http://www.galectintherapeutics.com/role/ComputationOfDilutedNetLossPerShareDetail" />
<roleRef xlink:type="simple" xlink:href="galt-20181231.xsd#DilutiveSharesWhichCouldExistPursuantToExerciseOfOutstandingStockInstrumentsAndWhichWereNotIncludedInCalculationDetail" roleURI="http://www.galectintherapeutics.com/role/DilutiveSharesWhichCouldExistPursuantToExerciseOfOutstandingStockInstrumentsAndWhichWereNotIncludedInCalculationDetail" />
<roleRef xlink:type="simple" xlink:href="galt-20181231.xsd#CommitmentsAndContingenciesAdditionalInformationDetail" roleURI="http://www.galectintherapeutics.com/role/CommitmentsAndContingenciesAdditionalInformationDetail" />
<roleRef xlink:type="simple" xlink:href="galt-20181231.xsd#FutureMinimumPaymentsOfOperatingLeaseDetail" roleURI="http://www.galectintherapeutics.com/role/FutureMinimumPaymentsOfOperatingLeaseDetail" />
<roleRef xlink:type="simple" xlink:href="galt-20181231.xsd#GalectinScienceLlcAdditionalInformationDetail" roleURI="http://www.galectintherapeutics.com/role/GalectinScienceLlcAdditionalInformationDetail" />
<roleRef xlink:type="simple" xlink:href="galt-20181231.xsd#IncomeTaxesAdditionalInformationDetail" roleURI="http://www.galectintherapeutics.com/role/IncomeTaxesAdditionalInformationDetail" />
<roleRef xlink:type="simple" xlink:href="galt-20181231.xsd#ComponentsOfNetDeferredTaxAssetsDetail" roleURI="http://www.galectintherapeutics.com/role/ComponentsOfNetDeferredTaxAssetsDetail" />
<roleRef xlink:type="simple" xlink:href="galt-20181231.xsd#PrimaryFactorsAffectingIncomeTaxRatesDetail" roleURI="http://www.galectintherapeutics.com/role/PrimaryFactorsAffectingIncomeTaxRatesDetail" />
<roleRef xlink:type="simple" xlink:href="galt-20181231.xsd#SubsequentEventsAdditionalInformationDetail" roleURI="http://www.galectintherapeutics.com/role/SubsequentEventsAdditionalInformationDetail" />
<arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/all" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#all" />
<arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-default" />
<arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-domain" />
<arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/domain-member" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#domain-member" />
<arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#hypercube-dimension" />
<arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/notAll" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#notAll" />
<definitionLink xlink:type="extended" xlink:role="http://www.galectintherapeutics.com/role/DocumentAndEntityInformation">
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_DocumentInformationLineItems" xlink:label="loc_dei_DocumentInformationLineItems_187664" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_EntityCommonStockSharesOutstanding" xlink:label="loc_dei_EntityCommonStockSharesOutstanding_322281" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_187664" xlink:to="loc_dei_EntityCommonStockSharesOutstanding_322281" use="optional" order="1" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_DocumentInformationTable" xlink:label="loc_dei_DocumentInformationTable_187667" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_dei_DocumentInformationLineItems_187664" xlink:to="loc_dei_DocumentInformationTable_187667" xbrldt:contextElement="segment" xbrldt:closed="true" use="optional" order="2" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementClassOfStockAxis" xlink:label="loc_us-gaap_StatementClassOfStockAxis_187666" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_dei_DocumentInformationTable_187667" xlink:to="loc_us-gaap_StatementClassOfStockAxis_187666" use="optional" order="3" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ClassOfStockDomain" xlink:label="loc_us-gaap_ClassOfStockDomain_187665" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_187666" xlink:to="loc_us-gaap_ClassOfStockDomain_187665" use="optional" order="4" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ClassOfStockDomain" xlink:label="loc_us-gaap_ClassOfStockDomain_187665_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_187666" xlink:to="loc_us-gaap_ClassOfStockDomain_187665_default" use="optional" order="5" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_DocumentType" xlink:label="loc_dei_DocumentType_187411" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_187664" xlink:to="loc_dei_DocumentType_187411" use="optional" order="6" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_AmendmentFlag" xlink:label="loc_dei_AmendmentFlag_187412" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_187664" xlink:to="loc_dei_AmendmentFlag_187412" use="optional" order="7" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_DocumentPeriodEndDate" xlink:label="loc_dei_DocumentPeriodEndDate_187421" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_187664" xlink:to="loc_dei_DocumentPeriodEndDate_187421" use="optional" order="8" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_DocumentFiscalYearFocus" xlink:label="loc_dei_DocumentFiscalYearFocus_187422" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_187664" xlink:to="loc_dei_DocumentFiscalYearFocus_187422" use="optional" order="9" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_DocumentFiscalPeriodFocus" xlink:label="loc_dei_DocumentFiscalPeriodFocus_187423" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_187664" xlink:to="loc_dei_DocumentFiscalPeriodFocus_187423" use="optional" order="10" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_EntityRegistrantName" xlink:label="loc_dei_EntityRegistrantName_187436" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_187664" xlink:to="loc_dei_EntityRegistrantName_187436" use="optional" order="11" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_EntityCentralIndexKey" xlink:label="loc_dei_EntityCentralIndexKey_187437" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_187664" xlink:to="loc_dei_EntityCentralIndexKey_187437" use="optional" order="12" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_CurrentFiscalYearEndDate" xlink:label="loc_dei_CurrentFiscalYearEndDate_187453" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_187664" xlink:to="loc_dei_CurrentFiscalYearEndDate_187453" use="optional" order="13" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_EntityWellKnownSeasonedIssuer" xlink:label="loc_dei_EntityWellKnownSeasonedIssuer_187455" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_187664" xlink:to="loc_dei_EntityWellKnownSeasonedIssuer_187455" use="optional" order="14" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_EntityVoluntaryFilers" xlink:label="loc_dei_EntityVoluntaryFilers_187456" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_187664" xlink:to="loc_dei_EntityVoluntaryFilers_187456" use="optional" order="15" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_EntityCurrentReportingStatus" xlink:label="loc_dei_EntityCurrentReportingStatus_187457" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_187664" xlink:to="loc_dei_EntityCurrentReportingStatus_187457" use="optional" order="16" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_EntityFilerCategory" xlink:label="loc_dei_EntityFilerCategory_187458" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_187664" xlink:to="loc_dei_EntityFilerCategory_187458" use="optional" order="17" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_EntityPublicFloat" xlink:label="loc_dei_EntityPublicFloat_187459" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_187664" xlink:to="loc_dei_EntityPublicFloat_187459" use="optional" order="18" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_TradingSymbol" xlink:label="loc_dei_TradingSymbol_187540" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_187664" xlink:to="loc_dei_TradingSymbol_187540" use="optional" order="19" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_EntityShellCompany" xlink:label="loc_dei_EntityShellCompany_187658" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_187664" xlink:to="loc_dei_EntityShellCompany_187658" use="optional" order="20" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_EntityEmergingGrowthCompany" xlink:label="loc_dei_EntityEmergingGrowthCompany_187659" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_187664" xlink:to="loc_dei_EntityEmergingGrowthCompany_187659" use="optional" order="21" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_EntitySmallBusiness" xlink:label="loc_dei_EntitySmallBusiness_187661" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_187664" xlink:to="loc_dei_EntitySmallBusiness_187661" use="optional" order="22" />
</definitionLink>
<definitionLink xlink:type="extended" xlink:role="http://www.galectintherapeutics.com/role/ConsolidatedBalanceSheets">
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementLineItems" xlink:label="loc_us-gaap_StatementLineItems_31" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_AssetsAbstract" xlink:label="loc_us-gaap_AssetsAbstract_24" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_31" xlink:to="loc_us-gaap_AssetsAbstract_24" use="optional" order="1" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_Assets" xlink:label="loc_us-gaap_Assets_37" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_24" xlink:to="loc_us-gaap_Assets_37" use="optional" order="2" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_AssetsCurrentAbstract" xlink:label="loc_us-gaap_AssetsCurrentAbstract_25" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_24" xlink:to="loc_us-gaap_AssetsCurrentAbstract_25" use="optional" order="3" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_AssetsCurrent" xlink:label="loc_us-gaap_AssetsCurrent_40" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsCurrentAbstract_25" xlink:to="loc_us-gaap_AssetsCurrent_40" use="optional" order="4" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_CashAndCashEquivalentsAtCarryingValue" xlink:label="loc_us-gaap_CashAndCashEquivalentsAtCarryingValue_38" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsCurrentAbstract_25" xlink:to="loc_us-gaap_CashAndCashEquivalentsAtCarryingValue_38" use="optional" order="5" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_PrepaidExpenseAndOtherAssetsCurrent" xlink:label="loc_us-gaap_PrepaidExpenseAndOtherAssetsCurrent_39" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsCurrentAbstract_25" xlink:to="loc_us-gaap_PrepaidExpenseAndOtherAssetsCurrent_39" use="optional" order="6" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_IntangibleAssetsNetExcludingGoodwill" xlink:label="loc_us-gaap_IntangibleAssetsNetExcludingGoodwill_36" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_24" xlink:to="loc_us-gaap_IntangibleAssetsNetExcludingGoodwill_36" use="optional" order="7" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_OtherAssetsNoncurrent" xlink:label="loc_us-gaap_OtherAssetsNoncurrent_35" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_24" xlink:to="loc_us-gaap_OtherAssetsNoncurrent_35" use="optional" order="8" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_PropertyPlantAndEquipmentNet" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentNet_34" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_24" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentNet_34" use="optional" order="9" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_LiabilitiesAndStockholdersEquityAbstract" xlink:label="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_28" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_31" xlink:to="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_28" use="optional" order="10" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_CommitmentsAndContingencies" xlink:label="loc_us-gaap_CommitmentsAndContingencies_48" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_28" xlink:to="loc_us-gaap_CommitmentsAndContingencies_48" use="optional" order="11" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_Liabilities" xlink:label="loc_us-gaap_Liabilities_47" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_28" xlink:to="loc_us-gaap_Liabilities_47" use="optional" order="12" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_LiabilitiesAndStockholdersEquity" xlink:label="loc_us-gaap_LiabilitiesAndStockholdersEquity_50" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_28" xlink:to="loc_us-gaap_LiabilitiesAndStockholdersEquity_50" use="optional" order="13" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_LiabilitiesCurrentAbstract" xlink:label="loc_us-gaap_LiabilitiesCurrentAbstract_29" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_28" xlink:to="loc_us-gaap_LiabilitiesCurrentAbstract_29" use="optional" order="14" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_AccountsPayableCurrent" xlink:label="loc_us-gaap_AccountsPayableCurrent_51" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_29" xlink:to="loc_us-gaap_AccountsPayableCurrent_51" use="optional" order="15" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_AccruedLiabilitiesCurrent" xlink:label="loc_us-gaap_AccruedLiabilitiesCurrent_52" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_29" xlink:to="loc_us-gaap_AccruedLiabilitiesCurrent_52" use="optional" order="16" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_DividendsPayableCurrent" xlink:label="loc_us-gaap_DividendsPayableCurrent_53" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_29" xlink:to="loc_us-gaap_DividendsPayableCurrent_53" use="optional" order="17" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_LiabilitiesCurrent" xlink:label="loc_us-gaap_LiabilitiesCurrent_54" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_29" xlink:to="loc_us-gaap_LiabilitiesCurrent_54" use="optional" order="18" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StockholdersEquityAbstract" xlink:label="loc_us-gaap_StockholdersEquityAbstract_33" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_28" xlink:to="loc_us-gaap_StockholdersEquityAbstract_33" use="optional" order="19" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_AdditionalPaidInCapital" xlink:label="loc_us-gaap_AdditionalPaidInCapital_58" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityAbstract_33" xlink:to="loc_us-gaap_AdditionalPaidInCapital_58" use="optional" order="20" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_CommonStockValue" xlink:label="loc_us-gaap_CommonStockValue_57" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityAbstract_33" xlink:to="loc_us-gaap_CommonStockValue_57" use="optional" order="21" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_PreferredStockValue" xlink:label="loc_us-gaap_PreferredStockValue_56" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityAbstract_33" xlink:to="loc_us-gaap_PreferredStockValue_56" use="optional" order="22" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_RetainedEarningsAccumulatedDeficit" xlink:label="loc_us-gaap_RetainedEarningsAccumulatedDeficit_59" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityAbstract_33" xlink:to="loc_us-gaap_RetainedEarningsAccumulatedDeficit_59" use="optional" order="23" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StockholdersEquity" xlink:label="loc_us-gaap_StockholdersEquity_60" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityAbstract_33" xlink:to="loc_us-gaap_StockholdersEquity_60" use="optional" order="24" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_UndesignatedCapitalStockValue" xlink:label="loc_galt_UndesignatedCapitalStockValue_55" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityAbstract_33" xlink:to="loc_galt_UndesignatedCapitalStockValue_55" use="optional" order="25" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_TemporaryEquityCarryingAmountAttributableToParent" xlink:label="loc_us-gaap_TemporaryEquityCarryingAmountAttributableToParent_49" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_28" xlink:to="loc_us-gaap_TemporaryEquityCarryingAmountAttributableToParent_49" use="optional" order="26" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementTable" xlink:label="loc_us-gaap_StatementTable_32" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_31" xlink:to="loc_us-gaap_StatementTable_32" xbrldt:contextElement="segment" xbrldt:closed="true" use="optional" order="27" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_LegalEntityAxis" xlink:label="loc_dei_LegalEntityAxis_27" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_32" xlink:to="loc_dei_LegalEntityAxis_27" use="optional" order="28" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_46" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_27" xlink:to="loc_dei_EntityDomain_46" use="optional" order="29" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_46_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_27" xlink:to="loc_dei_EntityDomain_46_default" use="optional" order="30" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementClassOfStockAxis" xlink:label="loc_us-gaap_StatementClassOfStockAxis_30" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_32" xlink:to="loc_us-gaap_StatementClassOfStockAxis_30" use="optional" order="31" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ClassOfStockDomain" xlink:label="loc_us-gaap_ClassOfStockDomain_26" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_30" xlink:to="loc_us-gaap_ClassOfStockDomain_26" use="optional" order="32" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ClassOfStockDomain" xlink:label="loc_us-gaap_ClassOfStockDomain_26_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_30" xlink:to="loc_us-gaap_ClassOfStockDomain_26_default" use="optional" order="33" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_SeriesAConvertiblePreferredStockMember" xlink:label="loc_galt_SeriesAConvertiblePreferredStockMember_42" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_26" xlink:to="loc_galt_SeriesAConvertiblePreferredStockMember_42" use="optional" order="34" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_SeriesBOneConvertiblePreferredStockMember" xlink:label="loc_galt_SeriesBOneConvertiblePreferredStockMember_43" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_26" xlink:to="loc_galt_SeriesBOneConvertiblePreferredStockMember_43" use="optional" order="35" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_SeriesBThreeConvertiblePreferredStockMember" xlink:label="loc_galt_SeriesBThreeConvertiblePreferredStockMember_45" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_26" xlink:to="loc_galt_SeriesBThreeConvertiblePreferredStockMember_45" use="optional" order="36" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_SeriesBTwoConvertiblePreferredStockMember" xlink:label="loc_galt_SeriesBTwoConvertiblePreferredStockMember_44" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_26" xlink:to="loc_galt_SeriesBTwoConvertiblePreferredStockMember_44" use="optional" order="37" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SeriesCPreferredStockMember" xlink:label="loc_us-gaap_SeriesCPreferredStockMember_41" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_26" xlink:to="loc_us-gaap_SeriesCPreferredStockMember_41" use="optional" order="38" />
</definitionLink>
<definitionLink xlink:type="extended" xlink:role="http://www.galectintherapeutics.com/role/ConsolidatedBalanceSheetsParenthetical">
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementLineItems" xlink:label="loc_us-gaap_StatementLineItems_66" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_CommonStockParOrStatedValuePerShare" xlink:label="loc_us-gaap_CommonStockParOrStatedValuePerShare_86" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_66" xlink:to="loc_us-gaap_CommonStockParOrStatedValuePerShare_86" use="optional" order="1" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_CommonStockSharesAuthorized" xlink:label="loc_us-gaap_CommonStockSharesAuthorized_87" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_66" xlink:to="loc_us-gaap_CommonStockSharesAuthorized_87" use="optional" order="2" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_CommonStockSharesIssued" xlink:label="loc_us-gaap_CommonStockSharesIssued_88" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_66" xlink:to="loc_us-gaap_CommonStockSharesIssued_88" use="optional" order="3" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_CommonStockSharesOutstanding" xlink:label="loc_us-gaap_CommonStockSharesOutstanding_89" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_66" xlink:to="loc_us-gaap_CommonStockSharesOutstanding_89" use="optional" order="4" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_PreferredStockLiquidationPreferenceValue" xlink:label="loc_us-gaap_PreferredStockLiquidationPreferenceValue_85" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_66" xlink:to="loc_us-gaap_PreferredStockLiquidationPreferenceValue_85" use="optional" order="5" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_PreferredStockSharesAuthorized" xlink:label="loc_us-gaap_PreferredStockSharesAuthorized_82" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_66" xlink:to="loc_us-gaap_PreferredStockSharesAuthorized_82" use="optional" order="6" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_PreferredStockSharesIssued" xlink:label="loc_us-gaap_PreferredStockSharesIssued_83" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_66" xlink:to="loc_us-gaap_PreferredStockSharesIssued_83" use="optional" order="7" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_PreferredStockSharesOutstanding" xlink:label="loc_us-gaap_PreferredStockSharesOutstanding_84" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_66" xlink:to="loc_us-gaap_PreferredStockSharesOutstanding_84" use="optional" order="8" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementTable" xlink:label="loc_us-gaap_StatementTable_67" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_66" xlink:to="loc_us-gaap_StatementTable_67" xbrldt:contextElement="segment" xbrldt:closed="true" use="optional" order="9" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_LegalEntityAxis" xlink:label="loc_dei_LegalEntityAxis_64" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_67" xlink:to="loc_dei_LegalEntityAxis_64" use="optional" order="10" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_73" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_64" xlink:to="loc_dei_EntityDomain_73" use="optional" order="11" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_73_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_64" xlink:to="loc_dei_EntityDomain_73_default" use="optional" order="12" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementClassOfStockAxis" xlink:label="loc_us-gaap_StatementClassOfStockAxis_65" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_67" xlink:to="loc_us-gaap_StatementClassOfStockAxis_65" use="optional" order="13" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ClassOfStockDomain" xlink:label="loc_us-gaap_ClassOfStockDomain_63" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_65" xlink:to="loc_us-gaap_ClassOfStockDomain_63" use="optional" order="14" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ClassOfStockDomain" xlink:label="loc_us-gaap_ClassOfStockDomain_63_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_65" xlink:to="loc_us-gaap_ClassOfStockDomain_63_default" use="optional" order="15" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_SeriesAConvertiblePreferredStockMember" xlink:label="loc_galt_SeriesAConvertiblePreferredStockMember_69" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_63" xlink:to="loc_galt_SeriesAConvertiblePreferredStockMember_69" use="optional" order="16" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_SeriesBOneConvertiblePreferredStockMember" xlink:label="loc_galt_SeriesBOneConvertiblePreferredStockMember_70" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_63" xlink:to="loc_galt_SeriesBOneConvertiblePreferredStockMember_70" use="optional" order="17" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_SeriesBThreeConvertiblePreferredStockMember" xlink:label="loc_galt_SeriesBThreeConvertiblePreferredStockMember_72" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_63" xlink:to="loc_galt_SeriesBThreeConvertiblePreferredStockMember_72" use="optional" order="18" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_SeriesBTwoConvertiblePreferredStockMember" xlink:label="loc_galt_SeriesBTwoConvertiblePreferredStockMember_71" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_63" xlink:to="loc_galt_SeriesBTwoConvertiblePreferredStockMember_71" use="optional" order="19" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SeriesCPreferredStockMember" xlink:label="loc_us-gaap_SeriesCPreferredStockMember_68" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_63" xlink:to="loc_us-gaap_SeriesCPreferredStockMember_68" use="optional" order="20" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_TemporaryEquityAggregateAmountOfRedemptionRequirement" xlink:label="loc_us-gaap_TemporaryEquityAggregateAmountOfRedemptionRequirement_77" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_66" xlink:to="loc_us-gaap_TemporaryEquityAggregateAmountOfRedemptionRequirement_77" use="optional" order="21" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_TemporaryEquityLiquidationPreference" xlink:label="loc_us-gaap_TemporaryEquityLiquidationPreference_78" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_66" xlink:to="loc_us-gaap_TemporaryEquityLiquidationPreference_78" use="optional" order="22" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_TemporaryEquitySharesAuthorized" xlink:label="loc_us-gaap_TemporaryEquitySharesAuthorized_74" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_66" xlink:to="loc_us-gaap_TemporaryEquitySharesAuthorized_74" use="optional" order="23" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_TemporaryEquitySharesIssued" xlink:label="loc_us-gaap_TemporaryEquitySharesIssued_75" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_66" xlink:to="loc_us-gaap_TemporaryEquitySharesIssued_75" use="optional" order="24" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_TemporaryEquitySharesOutstanding" xlink:label="loc_us-gaap_TemporaryEquitySharesOutstanding_76" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_66" xlink:to="loc_us-gaap_TemporaryEquitySharesOutstanding_76" use="optional" order="25" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_UndesignatedCapitalStockParValue" xlink:label="loc_galt_UndesignatedCapitalStockParValue_79" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_66" xlink:to="loc_galt_UndesignatedCapitalStockParValue_79" use="optional" order="26" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_UndesignatedCapitalStockSharesAuthorized" xlink:label="loc_galt_UndesignatedCapitalStockSharesAuthorized_80" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_66" xlink:to="loc_galt_UndesignatedCapitalStockSharesAuthorized_80" use="optional" order="27" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_UndesignatedCapitalStockSharesOutstanding" xlink:label="loc_galt_UndesignatedCapitalStockSharesOutstanding_81" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_66" xlink:to="loc_galt_UndesignatedCapitalStockSharesOutstanding_81" use="optional" order="28" />
</definitionLink>
<definitionLink xlink:type="extended" xlink:role="http://www.galectintherapeutics.com/role/ConsolidatedStatementsOfOperations">
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementLineItems" xlink:label="loc_us-gaap_StatementLineItems_95" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_EarningsPerShareBasicAndDiluted" xlink:label="loc_us-gaap_EarningsPerShareBasicAndDiluted_109" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_95" xlink:to="loc_us-gaap_EarningsPerShareBasicAndDiluted_109" use="optional" order="1" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_NetIncomeLoss" xlink:label="loc_us-gaap_NetIncomeLoss_105" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_95" xlink:to="loc_us-gaap_NetIncomeLoss_105" use="optional" order="2" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic" xlink:label="loc_us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic_108" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_95" xlink:to="loc_us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic_108" use="optional" order="3" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_NonoperatingIncomeExpenseAbstract" xlink:label="loc_us-gaap_NonoperatingIncomeExpenseAbstract_93" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_95" xlink:to="loc_us-gaap_NonoperatingIncomeExpenseAbstract_93" use="optional" order="4" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_InterestExpense" xlink:label="loc_us-gaap_InterestExpense_99" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NonoperatingIncomeExpenseAbstract_93" xlink:to="loc_us-gaap_InterestExpense_99" use="optional" order="5" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_InvestmentIncomeInterest" xlink:label="loc_us-gaap_InvestmentIncomeInterest_98" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NonoperatingIncomeExpenseAbstract_93" xlink:to="loc_us-gaap_InvestmentIncomeInterest_98" use="optional" order="6" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_NonoperatingIncomeExpense" xlink:label="loc_us-gaap_NonoperatingIncomeExpense_100" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NonoperatingIncomeExpenseAbstract_93" xlink:to="loc_us-gaap_NonoperatingIncomeExpense_100" use="optional" order="7" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_OperatingExpensesAbstract" xlink:label="loc_us-gaap_OperatingExpensesAbstract_94" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_95" xlink:to="loc_us-gaap_OperatingExpensesAbstract_94" use="optional" order="8" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_GeneralAndAdministrativeExpense" xlink:label="loc_us-gaap_GeneralAndAdministrativeExpense_102" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingExpensesAbstract_94" xlink:to="loc_us-gaap_GeneralAndAdministrativeExpense_102" use="optional" order="9" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_OperatingExpenses" xlink:label="loc_us-gaap_OperatingExpenses_103" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingExpensesAbstract_94" xlink:to="loc_us-gaap_OperatingExpenses_103" use="optional" order="10" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ResearchAndDevelopmentExpense" xlink:label="loc_us-gaap_ResearchAndDevelopmentExpense_101" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingExpensesAbstract_94" xlink:to="loc_us-gaap_ResearchAndDevelopmentExpense_101" use="optional" order="11" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_OperatingIncomeLoss" xlink:label="loc_us-gaap_OperatingIncomeLoss_104" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_95" xlink:to="loc_us-gaap_OperatingIncomeLoss_104" use="optional" order="12" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_PreferredStockDividendsIncomeStatementImpact" xlink:label="loc_us-gaap_PreferredStockDividendsIncomeStatementImpact_106" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_95" xlink:to="loc_us-gaap_PreferredStockDividendsIncomeStatementImpact_106" use="optional" order="13" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementTable" xlink:label="loc_us-gaap_StatementTable_96" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_95" xlink:to="loc_us-gaap_StatementTable_96" xbrldt:contextElement="segment" xbrldt:closed="true" use="optional" order="14" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_LegalEntityAxis" xlink:label="loc_dei_LegalEntityAxis_92" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_96" xlink:to="loc_dei_LegalEntityAxis_92" use="optional" order="15" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_97" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_92" xlink:to="loc_dei_EntityDomain_97" use="optional" order="16" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_97_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_92" xlink:to="loc_dei_EntityDomain_97_default" use="optional" order="17" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_WeightedAverageNumberOfShareOutstandingBasicAndDiluted" xlink:label="loc_us-gaap_WeightedAverageNumberOfShareOutstandingBasicAndDiluted_110" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_95" xlink:to="loc_us-gaap_WeightedAverageNumberOfShareOutstandingBasicAndDiluted_110" use="optional" order="18" />
</definitionLink>
<definitionLink xlink:type="extended" xlink:role="http://www.galectintherapeutics.com/role/ConsolidatedStatementsOfChangesInRedeemableConvertiblePreferredStockAndStockholdersEquityDeficit">
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementLineItems" xlink:label="loc_us-gaap_StatementLineItems_118" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue" xlink:label="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_159" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_118" xlink:to="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_159" use="optional" order="1" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_AdjustmentsToAdditionalPaidInCapitalWarrantIssued" xlink:label="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalWarrantIssued_158" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_118" xlink:to="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalWarrantIssued_158" use="optional" order="2" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_DividendsPreferredStock" xlink:label="loc_us-gaap_DividendsPreferredStock_140" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_118" xlink:to="loc_us-gaap_DividendsPreferredStock_140" use="optional" order="3" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_IssuanceOfStockAndWarrantsForServicesOrClaims" xlink:label="loc_us-gaap_IssuanceOfStockAndWarrantsForServicesOrClaims_153" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_118" xlink:to="loc_us-gaap_IssuanceOfStockAndWarrantsForServicesOrClaims_153" use="optional" order="4" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_IssuanceOfStockAndWarrantsForServicesOrClaimsShares" xlink:label="loc_galt_IssuanceOfStockAndWarrantsForServicesOrClaimsShares_155" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_118" xlink:to="loc_galt_IssuanceOfStockAndWarrantsForServicesOrClaimsShares_155" use="optional" order="5" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_NetIncomeLoss" xlink:label="loc_us-gaap_NetIncomeLoss_160" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_118" xlink:to="loc_us-gaap_NetIncomeLoss_160" use="optional" order="6" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_PreferredStockDividendsShares" xlink:label="loc_us-gaap_PreferredStockDividendsShares_141" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_118" xlink:to="loc_us-gaap_PreferredStockDividendsShares_141" use="optional" order="7" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SharesOutstanding" xlink:label="loc_us-gaap_SharesOutstanding_136" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_118" xlink:to="loc_us-gaap_SharesOutstanding_136" use="optional" order="8" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementTable" xlink:label="loc_us-gaap_StatementTable_119" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_118" xlink:to="loc_us-gaap_StatementTable_119" xbrldt:contextElement="segment" xbrldt:closed="true" use="optional" order="9" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_LegalEntityAxis" xlink:label="loc_dei_LegalEntityAxis_115" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_119" xlink:to="loc_dei_LegalEntityAxis_115" use="optional" order="10" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_135" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_115" xlink:to="loc_dei_EntityDomain_135" use="optional" order="11" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_135_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_115" xlink:to="loc_dei_EntityDomain_135_default" use="optional" order="12" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementClassOfStockAxis" xlink:label="loc_us-gaap_StatementClassOfStockAxis_116" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_119" xlink:to="loc_us-gaap_StatementClassOfStockAxis_116" use="optional" order="13" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ClassOfStockDomain" xlink:label="loc_us-gaap_ClassOfStockDomain_113" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_116" xlink:to="loc_us-gaap_ClassOfStockDomain_113" use="optional" order="14" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ClassOfStockDomain" xlink:label="loc_us-gaap_ClassOfStockDomain_113_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_116" xlink:to="loc_us-gaap_ClassOfStockDomain_113_default" use="optional" order="15" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SeriesAPreferredStockMember" xlink:label="loc_us-gaap_SeriesAPreferredStockMember_120" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_113" xlink:to="loc_us-gaap_SeriesAPreferredStockMember_120" use="optional" order="16" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_SeriesBOneConvertiblePreferredStockMember" xlink:label="loc_galt_SeriesBOneConvertiblePreferredStockMember_123" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_113" xlink:to="loc_galt_SeriesBOneConvertiblePreferredStockMember_123" use="optional" order="17" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_SeriesBTwoConvertiblePreferredStockMember" xlink:label="loc_galt_SeriesBTwoConvertiblePreferredStockMember_124" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_113" xlink:to="loc_galt_SeriesBTwoConvertiblePreferredStockMember_124" use="optional" order="18" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SeriesCPreferredStockMember" xlink:label="loc_us-gaap_SeriesCPreferredStockMember_121" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_113" xlink:to="loc_us-gaap_SeriesCPreferredStockMember_121" use="optional" order="19" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_StockBThreeConvertiblePreferredStockMember" xlink:label="loc_galt_StockBThreeConvertiblePreferredStockMember_122" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_113" xlink:to="loc_galt_StockBThreeConvertiblePreferredStockMember_122" use="optional" order="20" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementEquityComponentsAxis" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_117" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_119" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_117" use="optional" order="21" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_EquityComponentDomain" xlink:label="loc_us-gaap_EquityComponentDomain_114" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_117" xlink:to="loc_us-gaap_EquityComponentDomain_114" use="optional" order="22" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_EquityComponentDomain" xlink:label="loc_us-gaap_EquityComponentDomain_114_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_117" xlink:to="loc_us-gaap_EquityComponentDomain_114_default" use="optional" order="23" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_AdditionalPaidInCapitalMember" xlink:label="loc_us-gaap_AdditionalPaidInCapitalMember_133" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_114" xlink:to="loc_us-gaap_AdditionalPaidInCapitalMember_133" use="optional" order="24" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_CommonStockMember" xlink:label="loc_us-gaap_CommonStockMember_132" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_114" xlink:to="loc_us-gaap_CommonStockMember_132" use="optional" order="25" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_RetainedEarningsMember" xlink:label="loc_us-gaap_RetainedEarningsMember_134" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_114" xlink:to="loc_us-gaap_RetainedEarningsMember_134" use="optional" order="26" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_SeriesAOnePreferredStockMember" xlink:label="loc_galt_SeriesAOnePreferredStockMember_128" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_114" xlink:to="loc_galt_SeriesAOnePreferredStockMember_128" use="optional" order="27" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_SeriesBOnePreferredStockMember" xlink:label="loc_galt_SeriesBOnePreferredStockMember_129" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_114" xlink:to="loc_galt_SeriesBOnePreferredStockMember_129" use="optional" order="28" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_SeriesBThreePreferredStockMember" xlink:label="loc_galt_SeriesBThreePreferredStockMember_131" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_114" xlink:to="loc_galt_SeriesBThreePreferredStockMember_131" use="optional" order="29" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_SeriesBTwoPreferredStockMember" xlink:label="loc_galt_SeriesBTwoPreferredStockMember_130" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_114" xlink:to="loc_galt_SeriesBTwoPreferredStockMember_130" use="optional" order="30" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_StockAndWarrantsIssuedDuringPeriodSharesNewIssues" xlink:label="loc_galt_StockAndWarrantsIssuedDuringPeriodSharesNewIssues_148" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_118" xlink:to="loc_galt_StockAndWarrantsIssuedDuringPeriodSharesNewIssues_148" use="optional" order="31" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_StockAndWarrantsIssuedDuringPeriodValueNewIssues" xlink:label="loc_galt_StockAndWarrantsIssuedDuringPeriodValueNewIssues_146" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_118" xlink:to="loc_galt_StockAndWarrantsIssuedDuringPeriodValueNewIssues_146" use="optional" order="32" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StockholdersEquity" xlink:label="loc_us-gaap_StockholdersEquity_137" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_118" xlink:to="loc_us-gaap_StockholdersEquity_137" use="optional" order="33" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StockIssuedDuringPeriodSharesNewIssues" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesNewIssues_151" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_118" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesNewIssues_151" use="optional" order="34" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StockIssuedDuringPeriodSharesRestrictedStockAwardGross" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesRestrictedStockAwardGross_157" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_118" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesRestrictedStockAwardGross_157" use="optional" order="35" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StockIssuedDuringPeriodValueNewIssues" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueNewIssues_150" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_118" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueNewIssues_150" use="optional" order="36" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StockIssuedDuringPeriodValueRestrictedStockAwardGross" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueRestrictedStockAwardGross_156" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_118" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueRestrictedStockAwardGross_156" use="optional" order="37" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_TemporaryEquityCarryingAmountAttributableToParent" xlink:label="loc_us-gaap_TemporaryEquityCarryingAmountAttributableToParent_138" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_118" xlink:to="loc_us-gaap_TemporaryEquityCarryingAmountAttributableToParent_138" use="optional" order="38" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_TemporaryEquitySharesOutstanding" xlink:label="loc_us-gaap_TemporaryEquitySharesOutstanding_139" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_118" xlink:to="loc_us-gaap_TemporaryEquitySharesOutstanding_139" use="optional" order="39" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_320963" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_118" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_320963" use="optional" order="40" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised_320964" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_118" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised_320964" use="optional" order="41" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StockIssuedDuringPeriodValueConversionOfConvertibleSecurities" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueConversionOfConvertibleSecurities_320965" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_118" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueConversionOfConvertibleSecurities_320965" use="optional" order="42" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StockIssuedDuringPeriodSharesConversionOfConvertibleSecurities" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesConversionOfConvertibleSecurities_320967" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_118" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesConversionOfConvertibleSecurities_320967" use="optional" order="43" />
</definitionLink>
<definitionLink xlink:type="extended" xlink:role="http://www.galectintherapeutics.com/role/ConsolidatedStatementsOfCashFlows">
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementLineItems" xlink:label="loc_us-gaap_StatementLineItems_172" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_CashAndCashEquivalentsAtCarryingValue" xlink:label="loc_us-gaap_CashAndCashEquivalentsAtCarryingValue_189" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_172" xlink:to="loc_us-gaap_CashAndCashEquivalentsAtCarryingValue_189" use="optional" order="1" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_CashAndCashEquivalentsPeriodIncreaseDecrease" xlink:label="loc_us-gaap_CashAndCashEquivalentsPeriodIncreaseDecrease_188" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_172" xlink:to="loc_us-gaap_CashAndCashEquivalentsPeriodIncreaseDecrease_188" use="optional" order="2" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_CashFlowNoncashInvestingAndFinancingActivitiesDisclosureAbstract" xlink:label="loc_us-gaap_CashFlowNoncashInvestingAndFinancingActivitiesDisclosureAbstract_166" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_172" xlink:to="loc_us-gaap_CashFlowNoncashInvestingAndFinancingActivitiesDisclosureAbstract_166" use="optional" order="3" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_CommonStockPurchaseWarrantsIssuedRelatedWithLineOfCredit" xlink:label="loc_galt_CommonStockPurchaseWarrantsIssuedRelatedWithLineOfCredit_179" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CashFlowNoncashInvestingAndFinancingActivitiesDisclosureAbstract_166" xlink:to="loc_galt_CommonStockPurchaseWarrantsIssuedRelatedWithLineOfCredit_179" use="optional" order="4" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_SupplementalDividendsOnConvertiblePreferredStock" xlink:label="loc_galt_SupplementalDividendsOnConvertiblePreferredStock_178" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CashFlowNoncashInvestingAndFinancingActivitiesDisclosureAbstract_166" xlink:to="loc_galt_SupplementalDividendsOnConvertiblePreferredStock_178" use="optional" order="5" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract" xlink:label="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_169" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_172" xlink:to="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_169" use="optional" order="6" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_NetCashProvidedByUsedInFinancingActivities" xlink:label="loc_us-gaap_NetCashProvidedByUsedInFinancingActivities_184" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_169" xlink:to="loc_us-gaap_NetCashProvidedByUsedInFinancingActivities_184" use="optional" order="7" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ProceedsFromIssuanceOfCommonStock" xlink:label="loc_us-gaap_ProceedsFromIssuanceOfCommonStock_183" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_169" xlink:to="loc_us-gaap_ProceedsFromIssuanceOfCommonStock_183" use="optional" order="8" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract" xlink:label="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract_170" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_172" xlink:to="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract_170" use="optional" order="9" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_NetCashProvidedByUsedInInvestingActivities" xlink:label="loc_us-gaap_NetCashProvidedByUsedInInvestingActivities_185" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract_170" xlink:to="loc_us-gaap_NetCashProvidedByUsedInInvestingActivities_185" use="optional" order="10" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementTable" xlink:label="loc_us-gaap_StatementTable_173" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_172" xlink:to="loc_us-gaap_StatementTable_173" xbrldt:contextElement="segment" xbrldt:closed="true" use="optional" order="11" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_LegalEntityAxis" xlink:label="loc_dei_LegalEntityAxis_168" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_173" xlink:to="loc_dei_LegalEntityAxis_168" use="optional" order="12" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_182" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_168" xlink:to="loc_dei_EntityDomain_182" use="optional" order="13" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_182_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_168" xlink:to="loc_dei_EntityDomain_182_default" use="optional" order="14" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract" xlink:label="loc_us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract_171" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_172" xlink:to="loc_us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract_171" use="optional" order="15" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract" xlink:label="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_165" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract_171" xlink:to="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_165" use="optional" order="16" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_DepreciationDepletionAndAmortization" xlink:label="loc_us-gaap_DepreciationDepletionAndAmortization_174" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_165" xlink:to="loc_us-gaap_DepreciationDepletionAndAmortization_174" use="optional" order="17" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_IncreaseDecreaseInOperatingCapitalAbstract" xlink:label="loc_us-gaap_IncreaseDecreaseInOperatingCapitalAbstract_167" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_165" xlink:to="loc_us-gaap_IncreaseDecreaseInOperatingCapitalAbstract_167" use="optional" order="18" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_IncreaseDecreaseInAccountsPayableAndAccruedLiabilities" xlink:label="loc_us-gaap_IncreaseDecreaseInAccountsPayableAndAccruedLiabilities_181" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInOperatingCapitalAbstract_167" xlink:to="loc_us-gaap_IncreaseDecreaseInAccountsPayableAndAccruedLiabilities_181" use="optional" order="19" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_IncreaseDecreaseInPrepaidDeferredExpenseAndOtherAssets" xlink:label="loc_us-gaap_IncreaseDecreaseInPrepaidDeferredExpenseAndOtherAssets_180" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInOperatingCapitalAbstract_167" xlink:to="loc_us-gaap_IncreaseDecreaseInPrepaidDeferredExpenseAndOtherAssets_180" use="optional" order="20" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_IssuanceOfStockAndWarrantsForServicesOrClaims" xlink:label="loc_us-gaap_IssuanceOfStockAndWarrantsForServicesOrClaims_176" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_165" xlink:to="loc_us-gaap_IssuanceOfStockAndWarrantsForServicesOrClaims_176" use="optional" order="21" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_PaidInKindInterest" xlink:label="loc_us-gaap_PaidInKindInterest_177" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_165" xlink:to="loc_us-gaap_PaidInKindInterest_177" use="optional" order="22" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensation" xlink:label="loc_us-gaap_ShareBasedCompensation_175" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_165" xlink:to="loc_us-gaap_ShareBasedCompensation_175" use="optional" order="23" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_NetIncomeLoss" xlink:label="loc_us-gaap_NetIncomeLoss_186" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract_171" xlink:to="loc_us-gaap_NetIncomeLoss_186" use="optional" order="24" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_NetCashProvidedByUsedInOperatingActivities" xlink:label="loc_us-gaap_NetCashProvidedByUsedInOperatingActivities_187" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract_171" xlink:to="loc_us-gaap_NetCashProvidedByUsedInOperatingActivities_187" use="optional" order="25" />
</definitionLink>
<definitionLink xlink:type="extended" xlink:role="http://www.galectintherapeutics.com/role/NatureOfBusinessAndBasisOfPresentation">
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementLineItems" xlink:label="loc_us-gaap_StatementLineItems_771" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_BasisOfAccounting" xlink:label="loc_us-gaap_BasisOfAccounting_774" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_771" xlink:to="loc_us-gaap_BasisOfAccounting_774" use="optional" order="1" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementTable" xlink:label="loc_us-gaap_StatementTable_772" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_771" xlink:to="loc_us-gaap_StatementTable_772" xbrldt:contextElement="segment" xbrldt:closed="true" use="optional" order="2" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_LegalEntityAxis" xlink:label="loc_dei_LegalEntityAxis_770" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_772" xlink:to="loc_dei_LegalEntityAxis_770" use="optional" order="3" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_773" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_770" xlink:to="loc_dei_EntityDomain_773" use="optional" order="4" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_773_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_770" xlink:to="loc_dei_EntityDomain_773_default" use="optional" order="5" />
</definitionLink>
<definitionLink xlink:type="extended" xlink:role="http://www.galectintherapeutics.com/role/SummaryOfSignificantAccountingPolicies">
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementLineItems" xlink:label="loc_us-gaap_StatementLineItems_778" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SignificantAccountingPoliciesTextBlock" xlink:label="loc_us-gaap_SignificantAccountingPoliciesTextBlock_781" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_778" xlink:to="loc_us-gaap_SignificantAccountingPoliciesTextBlock_781" use="optional" order="1" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementTable" xlink:label="loc_us-gaap_StatementTable_779" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_778" xlink:to="loc_us-gaap_StatementTable_779" xbrldt:contextElement="segment" xbrldt:closed="true" use="optional" order="2" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_LegalEntityAxis" xlink:label="loc_dei_LegalEntityAxis_777" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_779" xlink:to="loc_dei_LegalEntityAxis_777" use="optional" order="3" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_780" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_777" xlink:to="loc_dei_EntityDomain_780" use="optional" order="4" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_780_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_777" xlink:to="loc_dei_EntityDomain_780_default" use="optional" order="5" />
</definitionLink>
<definitionLink xlink:type="extended" xlink:role="http://www.galectintherapeutics.com/role/PropertyAndEquipment">
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementLineItems" xlink:label="loc_us-gaap_StatementLineItems_807" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_PropertyPlantAndEquipmentDisclosureTextBlock" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentDisclosureTextBlock_810" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_807" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentDisclosureTextBlock_810" use="optional" order="1" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementTable" xlink:label="loc_us-gaap_StatementTable_808" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_807" xlink:to="loc_us-gaap_StatementTable_808" xbrldt:contextElement="segment" xbrldt:closed="true" use="optional" order="2" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_LegalEntityAxis" xlink:label="loc_dei_LegalEntityAxis_806" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_808" xlink:to="loc_dei_LegalEntityAxis_806" use="optional" order="3" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_809" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_806" xlink:to="loc_dei_EntityDomain_809" use="optional" order="4" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_809_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_806" xlink:to="loc_dei_EntityDomain_809_default" use="optional" order="5" />
</definitionLink>
<definitionLink xlink:type="extended" xlink:role="http://www.galectintherapeutics.com/role/AccruedExpenses">
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementLineItems" xlink:label="loc_us-gaap_StatementLineItems_821" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_AccountsPayableAndAccruedLiabilitiesDisclosureTextBlock" xlink:label="loc_us-gaap_AccountsPayableAndAccruedLiabilitiesDisclosureTextBlock_824" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_821" xlink:to="loc_us-gaap_AccountsPayableAndAccruedLiabilitiesDisclosureTextBlock_824" use="optional" order="1" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementTable" xlink:label="loc_us-gaap_StatementTable_822" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_821" xlink:to="loc_us-gaap_StatementTable_822" xbrldt:contextElement="segment" xbrldt:closed="true" use="optional" order="2" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_LegalEntityAxis" xlink:label="loc_dei_LegalEntityAxis_820" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_822" xlink:to="loc_dei_LegalEntityAxis_820" use="optional" order="3" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_823" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_820" xlink:to="loc_dei_EntityDomain_823" use="optional" order="4" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_823_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_820" xlink:to="loc_dei_EntityDomain_823_default" use="optional" order="5" />
</definitionLink>
<definitionLink xlink:type="extended" xlink:role="http://www.galectintherapeutics.com/role/StockholdersEquity">
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementLineItems" xlink:label="loc_us-gaap_StatementLineItems_835" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementTable" xlink:label="loc_us-gaap_StatementTable_836" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_835" xlink:to="loc_us-gaap_StatementTable_836" xbrldt:contextElement="segment" xbrldt:closed="true" use="optional" order="1" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_LegalEntityAxis" xlink:label="loc_dei_LegalEntityAxis_834" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_836" xlink:to="loc_dei_LegalEntityAxis_834" use="optional" order="2" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_837" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_834" xlink:to="loc_dei_EntityDomain_837" use="optional" order="3" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_837_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_834" xlink:to="loc_dei_EntityDomain_837_default" use="optional" order="4" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StockholdersEquityNoteDisclosureTextBlock" xlink:label="loc_us-gaap_StockholdersEquityNoteDisclosureTextBlock_838" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_835" xlink:to="loc_us-gaap_StockholdersEquityNoteDisclosureTextBlock_838" use="optional" order="5" />
</definitionLink>
<definitionLink xlink:type="extended" xlink:role="http://www.galectintherapeutics.com/role/Warrants">
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementLineItems" xlink:label="loc_us-gaap_StatementLineItems_849" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementTable" xlink:label="loc_us-gaap_StatementTable_850" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_849" xlink:to="loc_us-gaap_StatementTable_850" xbrldt:contextElement="segment" xbrldt:closed="true" use="optional" order="1" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_LegalEntityAxis" xlink:label="loc_dei_LegalEntityAxis_848" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_850" xlink:to="loc_dei_LegalEntityAxis_848" use="optional" order="2" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_851" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_848" xlink:to="loc_dei_EntityDomain_851" use="optional" order="3" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_851_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_848" xlink:to="loc_dei_EntityDomain_851_default" use="optional" order="4" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_StockWarrantsTextBlock" xlink:label="loc_galt_StockWarrantsTextBlock_852" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_849" xlink:to="loc_galt_StockWarrantsTextBlock_852" use="optional" order="5" />
</definitionLink>
<definitionLink xlink:type="extended" xlink:role="http://www.galectintherapeutics.com/role/StockBasedCompensation">
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementLineItems" xlink:label="loc_us-gaap_StatementLineItems_863" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_DisclosureOfCompensationRelatedCostsShareBasedPaymentsTextBlock" xlink:label="loc_us-gaap_DisclosureOfCompensationRelatedCostsShareBasedPaymentsTextBlock_866" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_863" xlink:to="loc_us-gaap_DisclosureOfCompensationRelatedCostsShareBasedPaymentsTextBlock_866" use="optional" order="1" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementTable" xlink:label="loc_us-gaap_StatementTable_864" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_863" xlink:to="loc_us-gaap_StatementTable_864" xbrldt:contextElement="segment" xbrldt:closed="true" use="optional" order="2" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_LegalEntityAxis" xlink:label="loc_dei_LegalEntityAxis_862" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_864" xlink:to="loc_dei_LegalEntityAxis_862" use="optional" order="3" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_865" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_862" xlink:to="loc_dei_EntityDomain_865" use="optional" order="4" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_865_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_862" xlink:to="loc_dei_EntityDomain_865_default" use="optional" order="5" />
</definitionLink>
<definitionLink xlink:type="extended" xlink:role="http://www.galectintherapeutics.com/role/LineOfCredit">
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementLineItems" xlink:label="loc_us-gaap_StatementLineItems_880" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_DebtDisclosureTextBlock" xlink:label="loc_us-gaap_DebtDisclosureTextBlock_883" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_880" xlink:to="loc_us-gaap_DebtDisclosureTextBlock_883" use="optional" order="1" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementTable" xlink:label="loc_us-gaap_StatementTable_881" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_880" xlink:to="loc_us-gaap_StatementTable_881" xbrldt:contextElement="segment" xbrldt:closed="true" use="optional" order="2" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_LegalEntityAxis" xlink:label="loc_dei_LegalEntityAxis_879" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_881" xlink:to="loc_dei_LegalEntityAxis_879" use="optional" order="3" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_882" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_879" xlink:to="loc_dei_EntityDomain_882" use="optional" order="4" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_882_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_879" xlink:to="loc_dei_EntityDomain_882_default" use="optional" order="5" />
</definitionLink>
<definitionLink xlink:type="extended" xlink:role="http://www.galectintherapeutics.com/role/LossPerShare">
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementLineItems" xlink:label="loc_us-gaap_StatementLineItems_887" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_EarningsPerShareTextBlock" xlink:label="loc_us-gaap_EarningsPerShareTextBlock_890" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_887" xlink:to="loc_us-gaap_EarningsPerShareTextBlock_890" use="optional" order="1" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementTable" xlink:label="loc_us-gaap_StatementTable_888" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_887" xlink:to="loc_us-gaap_StatementTable_888" xbrldt:contextElement="segment" xbrldt:closed="true" use="optional" order="2" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_LegalEntityAxis" xlink:label="loc_dei_LegalEntityAxis_886" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_888" xlink:to="loc_dei_LegalEntityAxis_886" use="optional" order="3" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_889" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_886" xlink:to="loc_dei_EntityDomain_889" use="optional" order="4" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_889_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_886" xlink:to="loc_dei_EntityDomain_889_default" use="optional" order="5" />
</definitionLink>
<definitionLink xlink:type="extended" xlink:role="http://www.galectintherapeutics.com/role/CommitmentsAndContingencies">
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementLineItems" xlink:label="loc_us-gaap_StatementLineItems_902" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_CommitmentsAndContingenciesDisclosureTextBlock" xlink:label="loc_us-gaap_CommitmentsAndContingenciesDisclosureTextBlock_905" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_902" xlink:to="loc_us-gaap_CommitmentsAndContingenciesDisclosureTextBlock_905" use="optional" order="1" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementTable" xlink:label="loc_us-gaap_StatementTable_903" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_902" xlink:to="loc_us-gaap_StatementTable_903" xbrldt:contextElement="segment" xbrldt:closed="true" use="optional" order="2" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_LegalEntityAxis" xlink:label="loc_dei_LegalEntityAxis_901" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_903" xlink:to="loc_dei_LegalEntityAxis_901" use="optional" order="3" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_904" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_901" xlink:to="loc_dei_EntityDomain_904" use="optional" order="4" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_904_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_901" xlink:to="loc_dei_EntityDomain_904_default" use="optional" order="5" />
</definitionLink>
<definitionLink xlink:type="extended" xlink:role="http://www.galectintherapeutics.com/role/GalectinSciencesLlc">
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementLineItems" xlink:label="loc_us-gaap_StatementLineItems_916" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_InvestmentsInAndAdvancesToAffiliatesScheduleOfInvestmentsTextBlock" xlink:label="loc_us-gaap_InvestmentsInAndAdvancesToAffiliatesScheduleOfInvestmentsTextBlock_919" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_916" xlink:to="loc_us-gaap_InvestmentsInAndAdvancesToAffiliatesScheduleOfInvestmentsTextBlock_919" use="optional" order="1" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementTable" xlink:label="loc_us-gaap_StatementTable_917" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_916" xlink:to="loc_us-gaap_StatementTable_917" xbrldt:contextElement="segment" xbrldt:closed="true" use="optional" order="2" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_LegalEntityAxis" xlink:label="loc_dei_LegalEntityAxis_915" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_917" xlink:to="loc_dei_LegalEntityAxis_915" use="optional" order="3" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_918" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_915" xlink:to="loc_dei_EntityDomain_918" use="optional" order="4" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_918_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_915" xlink:to="loc_dei_EntityDomain_918_default" use="optional" order="5" />
</definitionLink>
<definitionLink xlink:type="extended" xlink:role="http://www.galectintherapeutics.com/role/IncomeTaxes">
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementLineItems" xlink:label="loc_us-gaap_StatementLineItems_923" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_IncomeTaxDisclosureTextBlock" xlink:label="loc_us-gaap_IncomeTaxDisclosureTextBlock_926" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_923" xlink:to="loc_us-gaap_IncomeTaxDisclosureTextBlock_926" use="optional" order="1" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementTable" xlink:label="loc_us-gaap_StatementTable_924" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_923" xlink:to="loc_us-gaap_StatementTable_924" xbrldt:contextElement="segment" xbrldt:closed="true" use="optional" order="2" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_LegalEntityAxis" xlink:label="loc_dei_LegalEntityAxis_922" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_924" xlink:to="loc_dei_LegalEntityAxis_922" use="optional" order="3" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_925" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_922" xlink:to="loc_dei_EntityDomain_925" use="optional" order="4" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_925_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_922" xlink:to="loc_dei_EntityDomain_925_default" use="optional" order="5" />
</definitionLink>
<definitionLink xlink:type="extended" xlink:role="http://www.galectintherapeutics.com/role/SubsequentEvents">
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementLineItems" xlink:label="loc_us-gaap_StatementLineItems_320862" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementTable" xlink:label="loc_us-gaap_StatementTable_320855" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_320862" xlink:to="loc_us-gaap_StatementTable_320855" xbrldt:contextElement="segment" xbrldt:closed="true" use="optional" order="1" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_LegalEntityAxis" xlink:label="loc_dei_LegalEntityAxis_320858" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_320855" xlink:to="loc_dei_LegalEntityAxis_320858" use="optional" order="2" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_320860" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_320858" xlink:to="loc_dei_EntityDomain_320860" use="optional" order="3" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_320860_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_320858" xlink:to="loc_dei_EntityDomain_320860_default" use="optional" order="4" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SubsequentEventsTextBlock" xlink:label="loc_us-gaap_SubsequentEventsTextBlock_320864" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_320862" xlink:to="loc_us-gaap_SubsequentEventsTextBlock_320864" use="optional" order="5" />
</definitionLink>
<definitionLink xlink:type="extended" xlink:role="http://www.galectintherapeutics.com/role/SummaryOfSignificantAccountingPoliciesPolicies">
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementLineItems" xlink:label="loc_us-gaap_StatementLineItems_785" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_AccruedExpensePolicyTextBlock" xlink:label="loc_galt_AccruedExpensePolicyTextBlock_797" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_785" xlink:to="loc_galt_AccruedExpensePolicyTextBlock_797" use="optional" order="1" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_CashAndCashEquivalentsPolicyTextBlock" xlink:label="loc_us-gaap_CashAndCashEquivalentsPolicyTextBlock_791" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_785" xlink:to="loc_us-gaap_CashAndCashEquivalentsPolicyTextBlock_791" use="optional" order="2" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_CashAndCashEquivalentsRestrictedCashAndCashEquivalentsPolicy" xlink:label="loc_us-gaap_CashAndCashEquivalentsRestrictedCashAndCashEquivalentsPolicy_794" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_785" xlink:to="loc_us-gaap_CashAndCashEquivalentsRestrictedCashAndCashEquivalentsPolicy_794" use="optional" order="3" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_CompensationRelatedCostsPolicyTextBlock" xlink:label="loc_us-gaap_CompensationRelatedCostsPolicyTextBlock_802" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_785" xlink:to="loc_us-gaap_CompensationRelatedCostsPolicyTextBlock_802" use="optional" order="4" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ConcentrationRiskCreditRisk" xlink:label="loc_us-gaap_ConcentrationRiskCreditRisk_801" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_785" xlink:to="loc_us-gaap_ConcentrationRiskCreditRisk_801" use="optional" order="5" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ConsolidationPolicyTextBlock" xlink:label="loc_us-gaap_ConsolidationPolicyTextBlock_788" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_785" xlink:to="loc_us-gaap_ConsolidationPolicyTextBlock_788" use="optional" order="6" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_DerivativesPolicyTextBlock" xlink:label="loc_us-gaap_DerivativesPolicyTextBlock_798" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_785" xlink:to="loc_us-gaap_DerivativesPolicyTextBlock_798" use="optional" order="7" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FairValueMeasurementPolicyPolicyTextBlock" xlink:label="loc_us-gaap_FairValueMeasurementPolicyPolicyTextBlock_790" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_785" xlink:to="loc_us-gaap_FairValueMeasurementPolicyPolicyTextBlock_790" use="optional" order="8" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_GoodwillAndIntangibleAssetsIntangibleAssetsPolicy" xlink:label="loc_us-gaap_GoodwillAndIntangibleAssetsIntangibleAssetsPolicy_795" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_785" xlink:to="loc_us-gaap_GoodwillAndIntangibleAssetsIntangibleAssetsPolicy_795" use="optional" order="9" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ImpairmentOrDisposalOfLongLivedAssetsPolicyTextBlock" xlink:label="loc_us-gaap_ImpairmentOrDisposalOfLongLivedAssetsPolicyTextBlock_796" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_785" xlink:to="loc_us-gaap_ImpairmentOrDisposalOfLongLivedAssetsPolicyTextBlock_796" use="optional" order="10" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_IncomeTaxPolicyTextBlock" xlink:label="loc_us-gaap_IncomeTaxPolicyTextBlock_800" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_785" xlink:to="loc_us-gaap_IncomeTaxPolicyTextBlock_800" use="optional" order="11" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_NewAccountingPronouncementsPolicyPolicyTextBlock" xlink:label="loc_us-gaap_NewAccountingPronouncementsPolicyPolicyTextBlock_803" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_785" xlink:to="loc_us-gaap_NewAccountingPronouncementsPolicyPolicyTextBlock_803" use="optional" order="12" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_PrepaidExpensesAndOtherPolicyTextBlock" xlink:label="loc_galt_PrepaidExpensesAndOtherPolicyTextBlock_792" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_785" xlink:to="loc_galt_PrepaidExpensesAndOtherPolicyTextBlock_792" use="optional" order="13" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_PropertyPlantAndEquipmentPolicyTextBlock" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentPolicyTextBlock_793" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_785" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentPolicyTextBlock_793" use="optional" order="14" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ResearchAndDevelopmentExpensePolicy" xlink:label="loc_us-gaap_ResearchAndDevelopmentExpensePolicy_799" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_785" xlink:to="loc_us-gaap_ResearchAndDevelopmentExpensePolicy_799" use="optional" order="15" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementTable" xlink:label="loc_us-gaap_StatementTable_786" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_785" xlink:to="loc_us-gaap_StatementTable_786" xbrldt:contextElement="segment" xbrldt:closed="true" use="optional" order="16" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_LegalEntityAxis" xlink:label="loc_dei_LegalEntityAxis_784" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_786" xlink:to="loc_dei_LegalEntityAxis_784" use="optional" order="17" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_787" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_784" xlink:to="loc_dei_EntityDomain_787" use="optional" order="18" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_787_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_784" xlink:to="loc_dei_EntityDomain_787_default" use="optional" order="19" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_UseOfEstimates" xlink:label="loc_us-gaap_UseOfEstimates_789" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_785" xlink:to="loc_us-gaap_UseOfEstimates_789" use="optional" order="20" />
</definitionLink>
<definitionLink xlink:type="extended" xlink:role="http://www.galectintherapeutics.com/role/PropertyAndEquipmentTables">
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementLineItems" xlink:label="loc_us-gaap_StatementLineItems_814" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_PropertyPlantAndEquipmentTextBlock" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTextBlock_817" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_814" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTextBlock_817" use="optional" order="1" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementTable" xlink:label="loc_us-gaap_StatementTable_815" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_814" xlink:to="loc_us-gaap_StatementTable_815" xbrldt:contextElement="segment" xbrldt:closed="true" use="optional" order="2" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_LegalEntityAxis" xlink:label="loc_dei_LegalEntityAxis_813" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_815" xlink:to="loc_dei_LegalEntityAxis_813" use="optional" order="3" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_816" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_813" xlink:to="loc_dei_EntityDomain_816" use="optional" order="4" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_816_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_813" xlink:to="loc_dei_EntityDomain_816_default" use="optional" order="5" />
</definitionLink>
<definitionLink xlink:type="extended" xlink:role="http://www.galectintherapeutics.com/role/AccruedExpensesTables">
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementLineItems" xlink:label="loc_us-gaap_StatementLineItems_828" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ScheduleOfAccruedLiabilitiesTableTextBlock" xlink:label="loc_us-gaap_ScheduleOfAccruedLiabilitiesTableTextBlock_831" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_828" xlink:to="loc_us-gaap_ScheduleOfAccruedLiabilitiesTableTextBlock_831" use="optional" order="1" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementTable" xlink:label="loc_us-gaap_StatementTable_829" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_828" xlink:to="loc_us-gaap_StatementTable_829" xbrldt:contextElement="segment" xbrldt:closed="true" use="optional" order="2" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_LegalEntityAxis" xlink:label="loc_dei_LegalEntityAxis_827" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_829" xlink:to="loc_dei_LegalEntityAxis_827" use="optional" order="3" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_830" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_827" xlink:to="loc_dei_EntityDomain_830" use="optional" order="4" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_830_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_827" xlink:to="loc_dei_EntityDomain_830_default" use="optional" order="5" />
</definitionLink>
<definitionLink xlink:type="extended" xlink:role="http://www.galectintherapeutics.com/role/StockholdersEquityTables">
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementLineItems" xlink:label="loc_us-gaap_StatementLineItems_842" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_ScheduleOfEquityAndOtherInvestmentsTableTextBlock" xlink:label="loc_galt_ScheduleOfEquityAndOtherInvestmentsTableTextBlock_845" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_842" xlink:to="loc_galt_ScheduleOfEquityAndOtherInvestmentsTableTextBlock_845" use="optional" order="1" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementTable" xlink:label="loc_us-gaap_StatementTable_843" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_842" xlink:to="loc_us-gaap_StatementTable_843" xbrldt:contextElement="segment" xbrldt:closed="true" use="optional" order="2" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_LegalEntityAxis" xlink:label="loc_dei_LegalEntityAxis_841" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_843" xlink:to="loc_dei_LegalEntityAxis_841" use="optional" order="3" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_844" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_841" xlink:to="loc_dei_EntityDomain_844" use="optional" order="4" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_844_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_841" xlink:to="loc_dei_EntityDomain_844_default" use="optional" order="5" />
</definitionLink>
<definitionLink xlink:type="extended" xlink:role="http://www.galectintherapeutics.com/role/WarrantsTables">
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementLineItems" xlink:label="loc_us-gaap_StatementLineItems_856" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ScheduleOfStockholdersEquityNoteWarrantsOrRightsTextBlock" xlink:label="loc_us-gaap_ScheduleOfStockholdersEquityNoteWarrantsOrRightsTextBlock_859" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_856" xlink:to="loc_us-gaap_ScheduleOfStockholdersEquityNoteWarrantsOrRightsTextBlock_859" use="optional" order="1" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementTable" xlink:label="loc_us-gaap_StatementTable_857" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_856" xlink:to="loc_us-gaap_StatementTable_857" xbrldt:contextElement="segment" xbrldt:closed="true" use="optional" order="2" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_LegalEntityAxis" xlink:label="loc_dei_LegalEntityAxis_855" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_857" xlink:to="loc_dei_LegalEntityAxis_855" use="optional" order="3" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_858" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_855" xlink:to="loc_dei_EntityDomain_858" use="optional" order="4" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_858_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_855" xlink:to="loc_dei_EntityDomain_858_default" use="optional" order="5" />
</definitionLink>
<definitionLink xlink:type="extended" xlink:role="http://www.galectintherapeutics.com/role/StockBasedCompensationTables">
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementLineItems" xlink:label="loc_us-gaap_StatementLineItems_870" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTextBlock" xlink:label="loc_us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTextBlock_873" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_870" xlink:to="loc_us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTextBlock_873" use="optional" order="1" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ScheduleOfShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeTextBlock" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeTextBlock_876" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_870" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeTextBlock_876" use="optional" order="2" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ScheduleOfShareBasedCompensationStockOptionsActivityTableTextBlock" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationStockOptionsActivityTableTextBlock_875" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_870" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationStockOptionsActivityTableTextBlock_875" use="optional" order="3" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ScheduleOfShareBasedPaymentAwardStockOptionsValuationAssumptionsTableTextBlock" xlink:label="loc_us-gaap_ScheduleOfShareBasedPaymentAwardStockOptionsValuationAssumptionsTableTextBlock_874" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_870" xlink:to="loc_us-gaap_ScheduleOfShareBasedPaymentAwardStockOptionsValuationAssumptionsTableTextBlock_874" use="optional" order="4" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementTable" xlink:label="loc_us-gaap_StatementTable_871" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_870" xlink:to="loc_us-gaap_StatementTable_871" xbrldt:contextElement="segment" xbrldt:closed="true" use="optional" order="5" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_LegalEntityAxis" xlink:label="loc_dei_LegalEntityAxis_869" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_871" xlink:to="loc_dei_LegalEntityAxis_869" use="optional" order="6" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_872" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_869" xlink:to="loc_dei_EntityDomain_872" use="optional" order="7" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_872_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_869" xlink:to="loc_dei_EntityDomain_872_default" use="optional" order="8" />
</definitionLink>
<definitionLink xlink:type="extended" xlink:role="http://www.galectintherapeutics.com/role/LossPerShareTables">
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementLineItems" xlink:label="loc_us-gaap_StatementLineItems_894" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTextBlock" xlink:label="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTextBlock_898" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_894" xlink:to="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTextBlock_898" use="optional" order="1" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_ScheduleOfComputationOfBasicAndDilutedEarningsPerShareTableTextBlock" xlink:label="loc_galt_ScheduleOfComputationOfBasicAndDilutedEarningsPerShareTableTextBlock_897" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_894" xlink:to="loc_galt_ScheduleOfComputationOfBasicAndDilutedEarningsPerShareTableTextBlock_897" use="optional" order="2" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementTable" xlink:label="loc_us-gaap_StatementTable_895" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_894" xlink:to="loc_us-gaap_StatementTable_895" xbrldt:contextElement="segment" xbrldt:closed="true" use="optional" order="3" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_LegalEntityAxis" xlink:label="loc_dei_LegalEntityAxis_893" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_895" xlink:to="loc_dei_LegalEntityAxis_893" use="optional" order="4" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_896" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_893" xlink:to="loc_dei_EntityDomain_896" use="optional" order="5" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_896_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_893" xlink:to="loc_dei_EntityDomain_896_default" use="optional" order="6" />
</definitionLink>
<definitionLink xlink:type="extended" xlink:role="http://www.galectintherapeutics.com/role/CommitmentsAndContingenciesTables">
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementLineItems" xlink:label="loc_us-gaap_StatementLineItems_909" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_OperatingLeasesOfLesseeDisclosureTextBlock" xlink:label="loc_us-gaap_OperatingLeasesOfLesseeDisclosureTextBlock_912" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_909" xlink:to="loc_us-gaap_OperatingLeasesOfLesseeDisclosureTextBlock_912" use="optional" order="1" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementTable" xlink:label="loc_us-gaap_StatementTable_910" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_909" xlink:to="loc_us-gaap_StatementTable_910" xbrldt:contextElement="segment" xbrldt:closed="true" use="optional" order="2" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_LegalEntityAxis" xlink:label="loc_dei_LegalEntityAxis_908" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_910" xlink:to="loc_dei_LegalEntityAxis_908" use="optional" order="3" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_911" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_908" xlink:to="loc_dei_EntityDomain_911" use="optional" order="4" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_911_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_908" xlink:to="loc_dei_EntityDomain_911_default" use="optional" order="5" />
</definitionLink>
<definitionLink xlink:type="extended" xlink:role="http://www.galectintherapeutics.com/role/IncomeTaxesTables">
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementLineItems" xlink:label="loc_us-gaap_StatementLineItems_930" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ScheduleOfDeferredTaxAssetsAndLiabilitiesTableTextBlock" xlink:label="loc_us-gaap_ScheduleOfDeferredTaxAssetsAndLiabilitiesTableTextBlock_933" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_930" xlink:to="loc_us-gaap_ScheduleOfDeferredTaxAssetsAndLiabilitiesTableTextBlock_933" use="optional" order="1" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ScheduleOfEffectiveIncomeTaxRateReconciliationTableTextBlock" xlink:label="loc_us-gaap_ScheduleOfEffectiveIncomeTaxRateReconciliationTableTextBlock_934" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_930" xlink:to="loc_us-gaap_ScheduleOfEffectiveIncomeTaxRateReconciliationTableTextBlock_934" use="optional" order="2" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementTable" xlink:label="loc_us-gaap_StatementTable_931" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_930" xlink:to="loc_us-gaap_StatementTable_931" xbrldt:contextElement="segment" xbrldt:closed="true" use="optional" order="3" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_LegalEntityAxis" xlink:label="loc_dei_LegalEntityAxis_929" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_931" xlink:to="loc_dei_LegalEntityAxis_929" use="optional" order="4" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_932" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_929" xlink:to="loc_dei_EntityDomain_932" use="optional" order="5" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_932_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_929" xlink:to="loc_dei_EntityDomain_932_default" use="optional" order="6" />
</definitionLink>
<definitionLink xlink:type="extended" xlink:role="http://www.galectintherapeutics.com/role/NatureOfBusinessAndBasisOfPresentationAdditionalInformationDetail">
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_BasisOfPresentationLineItems" xlink:label="loc_galt_BasisOfPresentationLineItems_193" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_BasisOfPresentationTable" xlink:label="loc_galt_BasisOfPresentationTable_194" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_galt_BasisOfPresentationLineItems_193" xlink:to="loc_galt_BasisOfPresentationTable_194" xbrldt:contextElement="segment" xbrldt:closed="true" use="optional" order="1" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_CreditFacilityAxis" xlink:label="loc_us-gaap_CreditFacilityAxis_195" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_galt_BasisOfPresentationTable_194" xlink:to="loc_us-gaap_CreditFacilityAxis_195" use="optional" order="2" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_CreditFacilityDomain" xlink:label="loc_us-gaap_CreditFacilityDomain_196" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_CreditFacilityAxis_195" xlink:to="loc_us-gaap_CreditFacilityDomain_196" use="optional" order="3" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_CreditFacilityDomain" xlink:label="loc_us-gaap_CreditFacilityDomain_196_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_CreditFacilityAxis_195" xlink:to="loc_us-gaap_CreditFacilityDomain_196_default" use="optional" order="4" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SubsequentEventTypeAxis" xlink:label="loc_us-gaap_SubsequentEventTypeAxis_198" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_galt_BasisOfPresentationTable_194" xlink:to="loc_us-gaap_SubsequentEventTypeAxis_198" use="optional" order="5" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SubsequentEventTypeDomain" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_199" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_SubsequentEventTypeAxis_198" xlink:to="loc_us-gaap_SubsequentEventTypeDomain_199" use="optional" order="6" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SubsequentEventTypeDomain" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_199_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_SubsequentEventTypeAxis_198" xlink:to="loc_us-gaap_SubsequentEventTypeDomain_199_default" use="optional" order="7" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SubsequentEventMember" xlink:label="loc_us-gaap_SubsequentEventMember_207" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventTypeDomain_199" xlink:to="loc_us-gaap_SubsequentEventMember_207" use="optional" order="8" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_TitleOfIndividualAxis" xlink:label="loc_us-gaap_TitleOfIndividualAxis_200" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_galt_BasisOfPresentationTable_194" xlink:to="loc_us-gaap_TitleOfIndividualAxis_200" use="optional" order="9" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_TitleOfIndividualWithRelationshipToEntityDomain" xlink:label="loc_us-gaap_TitleOfIndividualWithRelationshipToEntityDomain_201" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_TitleOfIndividualAxis_200" xlink:to="loc_us-gaap_TitleOfIndividualWithRelationshipToEntityDomain_201" use="optional" order="10" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_TitleOfIndividualWithRelationshipToEntityDomain" xlink:label="loc_us-gaap_TitleOfIndividualWithRelationshipToEntityDomain_201_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_TitleOfIndividualAxis_200" xlink:to="loc_us-gaap_TitleOfIndividualWithRelationshipToEntityDomain_201_default" use="optional" order="11" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_TypeOfArrangementAxis" xlink:label="loc_us-gaap_TypeOfArrangementAxis_321310" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_galt_BasisOfPresentationTable_194" xlink:to="loc_us-gaap_TypeOfArrangementAxis_321310" use="optional" order="12" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ArrangementsAndNonarrangementTransactionsMember" xlink:label="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_321311" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_TypeOfArrangementAxis_321310" xlink:to="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_321311" use="optional" order="13" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ArrangementsAndNonarrangementTransactionsMember" xlink:label="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_321311_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_TypeOfArrangementAxis_321310" xlink:to="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_321311_default" use="optional" order="14" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_TwoThousandNineteenAtMarketSalesAgreementMember" xlink:label="loc_galt_TwoThousandNineteenAtMarketSalesAgreementMember_321312" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_321311" xlink:to="loc_galt_TwoThousandNineteenAtMarketSalesAgreementMember_321312" use="optional" order="15" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_CashAndCashEquivalentsAtCarryingValue" xlink:label="loc_us-gaap_CashAndCashEquivalentsAtCarryingValue_202" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_galt_BasisOfPresentationLineItems_193" xlink:to="loc_us-gaap_CashAndCashEquivalentsAtCarryingValue_202" use="optional" order="16" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ProceedsFromIssuanceOfCommonStock" xlink:label="loc_us-gaap_ProceedsFromIssuanceOfCommonStock_321309" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_galt_BasisOfPresentationLineItems_193" xlink:to="loc_us-gaap_ProceedsFromIssuanceOfCommonStock_321309" use="optional" order="17" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_ExpectedCostsOfTrailAndGeneralOverhead" xlink:label="loc_galt_ExpectedCostsOfTrailAndGeneralOverhead_332948" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_galt_BasisOfPresentationLineItems_193" xlink:to="loc_galt_ExpectedCostsOfTrailAndGeneralOverhead_332948" use="optional" order="18" />
</definitionLink>
<definitionLink xlink:type="extended" xlink:role="http://www.galectintherapeutics.com/role/SummaryOfSignificantAccountingPoliciesAdditionalInformationDetail">
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_SignificantAccountingPoliciesLineItems" xlink:label="loc_galt_SignificantAccountingPoliciesLineItems_216" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_AmortizationOfIntangibleAssets" xlink:label="loc_us-gaap_AmortizationOfIntangibleAssets_229" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_galt_SignificantAccountingPoliciesLineItems_216" xlink:to="loc_us-gaap_AmortizationOfIntangibleAssets_229" use="optional" order="1" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_AssetsFairValueDisclosure" xlink:label="loc_us-gaap_AssetsFairValueDisclosure_223" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_galt_SignificantAccountingPoliciesLineItems_216" xlink:to="loc_us-gaap_AssetsFairValueDisclosure_223" use="optional" order="2" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_CashEquivalentsAtCarryingValue" xlink:label="loc_us-gaap_CashEquivalentsAtCarryingValue_225" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_galt_SignificantAccountingPoliciesLineItems_216" xlink:to="loc_us-gaap_CashEquivalentsAtCarryingValue_225" use="optional" order="3" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization_231" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_galt_SignificantAccountingPoliciesLineItems_216" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization_231" use="optional" order="4" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsGross" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsGross_230" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_galt_SignificantAccountingPoliciesLineItems_216" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsGross_230" use="optional" order="5" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetUsefulLife" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetUsefulLife_228" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_galt_SignificantAccountingPoliciesLineItems_216" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetUsefulLife_228" use="optional" order="6" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_LiabilitiesFairValueDisclosure" xlink:label="loc_us-gaap_LiabilitiesFairValueDisclosure_224" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_galt_SignificantAccountingPoliciesLineItems_216" xlink:to="loc_us-gaap_LiabilitiesFairValueDisclosure_224" use="optional" order="7" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_PropertyPlantAndEquipmentUsefulLife" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentUsefulLife_226" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_galt_SignificantAccountingPoliciesLineItems_216" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentUsefulLife_226" use="optional" order="8" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SecurityDeposit" xlink:label="loc_us-gaap_SecurityDeposit_227" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_galt_SignificantAccountingPoliciesLineItems_216" xlink:to="loc_us-gaap_SecurityDeposit_227" use="optional" order="9" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_SignificantAccountingPoliciesTable" xlink:label="loc_galt_SignificantAccountingPoliciesTable_217" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_galt_SignificantAccountingPoliciesLineItems_216" xlink:to="loc_galt_SignificantAccountingPoliciesTable_217" xbrldt:contextElement="segment" xbrldt:closed="true" use="optional" order="10" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis" xlink:label="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_211" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_galt_SignificantAccountingPoliciesTable_217" xlink:to="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_211" use="optional" order="11" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_212" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_211" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_212" use="optional" order="12" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_212_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_211" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_212_default" use="optional" order="13" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FairValueInputsLevel2Member" xlink:label="loc_us-gaap_FairValueInputsLevel2Member_218" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_212" xlink:to="loc_us-gaap_FairValueInputsLevel2Member_218" use="optional" order="14" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FairValueInputsLevel3Member" xlink:label="loc_us-gaap_FairValueInputsLevel3Member_219" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_212" xlink:to="loc_us-gaap_FairValueInputsLevel3Member_219" use="optional" order="15" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_LegalEntityAxis" xlink:label="loc_dei_LegalEntityAxis_213" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_galt_SignificantAccountingPoliciesTable_217" xlink:to="loc_dei_LegalEntityAxis_213" use="optional" order="16" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_220" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_213" xlink:to="loc_dei_EntityDomain_220" use="optional" order="17" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_220_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_213" xlink:to="loc_dei_EntityDomain_220_default" use="optional" order="18" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_PropertyPlantAndEquipmentByTypeAxis" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_214" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_galt_SignificantAccountingPoliciesTable_217" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_214" use="optional" order="19" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_215" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_214" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_215" use="optional" order="20" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_215_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_214" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_215_default" use="optional" order="21" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_ComputerAndOfficeEquipmentMember" xlink:label="loc_galt_ComputerAndOfficeEquipmentMember_221" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_215" xlink:to="loc_galt_ComputerAndOfficeEquipmentMember_221" use="optional" order="22" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FurnitureAndFixturesMember" xlink:label="loc_us-gaap_FurnitureAndFixturesMember_222" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_215" xlink:to="loc_us-gaap_FurnitureAndFixturesMember_222" use="optional" order="23" />
</definitionLink>
<definitionLink xlink:type="extended" xlink:role="http://www.galectintherapeutics.com/role/PropertyAndEquipmentDetail">
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_PropertyPlantAndEquipmentLineItems" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentLineItems_236" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment" xlink:label="loc_us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment_241" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_236" xlink:to="loc_us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment_241" use="optional" order="1" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_PropertyPlantAndEquipmentGross" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentGross_240" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_236" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentGross_240" use="optional" order="2" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_PropertyPlantAndEquipmentNet" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentNet_242" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_236" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentNet_242" use="optional" order="3" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ScheduleOfPropertyPlantAndEquipmentTable" xlink:label="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_238" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_236" xlink:to="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_238" xbrldt:contextElement="segment" xbrldt:closed="true" use="optional" order="4" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_LegalEntityAxis" xlink:label="loc_dei_LegalEntityAxis_234" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_238" xlink:to="loc_dei_LegalEntityAxis_234" use="optional" order="5" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_239" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_234" xlink:to="loc_dei_EntityDomain_239" use="optional" order="6" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_239_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_234" xlink:to="loc_dei_EntityDomain_239_default" use="optional" order="7" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_PropertyPlantAndEquipmentByTypeAxis" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_235" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_238" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_235" use="optional" order="8" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_237" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_235" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_237" use="optional" order="9" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_237_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_235" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_237_default" use="optional" order="10" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_ComputerAndOfficeEquipmentMember" xlink:label="loc_galt_ComputerAndOfficeEquipmentMember_244" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_237" xlink:to="loc_galt_ComputerAndOfficeEquipmentMember_244" use="optional" order="11" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FurnitureAndFixturesMember" xlink:label="loc_us-gaap_FurnitureAndFixturesMember_245" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_237" xlink:to="loc_us-gaap_FurnitureAndFixturesMember_245" use="optional" order="12" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_LeaseholdImprovementsMember" xlink:label="loc_us-gaap_LeaseholdImprovementsMember_243" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_237" xlink:to="loc_us-gaap_LeaseholdImprovementsMember_243" use="optional" order="13" />
</definitionLink>
<definitionLink xlink:type="extended" xlink:role="http://www.galectintherapeutics.com/role/PropertyAndEquipmentAdditionalInformationDetail">
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_PropertyPlantAndEquipmentLineItems" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentLineItems_249" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_DepreciationAndAmortization" xlink:label="loc_us-gaap_DepreciationAndAmortization_252" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_249" xlink:to="loc_us-gaap_DepreciationAndAmortization_252" use="optional" order="1" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ScheduleOfPropertyPlantAndEquipmentTable" xlink:label="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_250" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_249" xlink:to="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_250" xbrldt:contextElement="segment" xbrldt:closed="true" use="optional" order="2" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_LegalEntityAxis" xlink:label="loc_dei_LegalEntityAxis_248" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_250" xlink:to="loc_dei_LegalEntityAxis_248" use="optional" order="3" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_251" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_248" xlink:to="loc_dei_EntityDomain_251" use="optional" order="4" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_251_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_248" xlink:to="loc_dei_EntityDomain_251_default" use="optional" order="5" />
</definitionLink>
<definitionLink xlink:type="extended" xlink:role="http://www.galectintherapeutics.com/role/AccruedExpensesDetail">
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_ScheduleOfAccruedLiabilitiesLineItems" xlink:label="loc_galt_ScheduleOfAccruedLiabilitiesLineItems_256" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_AccruedLiabilitiesCurrent" xlink:label="loc_us-gaap_AccruedLiabilitiesCurrent_262" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_galt_ScheduleOfAccruedLiabilitiesLineItems_256" xlink:to="loc_us-gaap_AccruedLiabilitiesCurrent_262" use="optional" order="1" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_AccruedProfessionalFeesCurrent" xlink:label="loc_us-gaap_AccruedProfessionalFeesCurrent_259" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_galt_ScheduleOfAccruedLiabilitiesLineItems_256" xlink:to="loc_us-gaap_AccruedProfessionalFeesCurrent_259" use="optional" order="2" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_AccruedResearchAndDevelopmentCostsAndOther" xlink:label="loc_galt_AccruedResearchAndDevelopmentCostsAndOther_261" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_galt_ScheduleOfAccruedLiabilitiesLineItems_256" xlink:to="loc_galt_AccruedResearchAndDevelopmentCostsAndOther_261" use="optional" order="3" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_EmployeeRelatedLiabilitiesCurrent" xlink:label="loc_us-gaap_EmployeeRelatedLiabilitiesCurrent_260" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_galt_ScheduleOfAccruedLiabilitiesLineItems_256" xlink:to="loc_us-gaap_EmployeeRelatedLiabilitiesCurrent_260" use="optional" order="4" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_ScheduleOfAccruedLiabilitiesTable" xlink:label="loc_galt_ScheduleOfAccruedLiabilitiesTable_257" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_galt_ScheduleOfAccruedLiabilitiesLineItems_256" xlink:to="loc_galt_ScheduleOfAccruedLiabilitiesTable_257" xbrldt:contextElement="segment" xbrldt:closed="true" use="optional" order="5" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_LegalEntityAxis" xlink:label="loc_dei_LegalEntityAxis_255" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_galt_ScheduleOfAccruedLiabilitiesTable_257" xlink:to="loc_dei_LegalEntityAxis_255" use="optional" order="6" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_258" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_255" xlink:to="loc_dei_EntityDomain_258" use="optional" order="7" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_258_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_255" xlink:to="loc_dei_EntityDomain_258_default" use="optional" order="8" />
</definitionLink>
<definitionLink xlink:type="extended" xlink:role="http://www.galectintherapeutics.com/role/StockholdersEquityAdditionalInformationDetail">
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ClassOfStockLineItems" xlink:label="loc_us-gaap_ClassOfStockLineItems_267" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_AdjustmentsToAdditionalPaidInCapitalConvertibleDebtWithConversionFeature" xlink:label="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalConvertibleDebtWithConversionFeature_361" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_267" xlink:to="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalConvertibleDebtWithConversionFeature_361" use="optional" order="1" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_AggregateOfferingPrice" xlink:label="loc_galt_AggregateOfferingPrice_307" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_267" xlink:to="loc_galt_AggregateOfferingPrice_307" use="optional" order="2" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1" xlink:label="loc_us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1_313" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_267" xlink:to="loc_us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1_313" use="optional" order="3" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_ClassOfWarrantOrRightIssuedInPeriod" xlink:label="loc_galt_ClassOfWarrantOrRightIssuedInPeriod_312" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_267" xlink:to="loc_galt_ClassOfWarrantOrRightIssuedInPeriod_312" use="optional" order="4" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_ClosingStockPriceOfCommonStock" xlink:label="loc_galt_ClosingStockPriceOfCommonStock_332" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_267" xlink:to="loc_galt_ClosingStockPriceOfCommonStock_332" use="optional" order="5" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_CommissionsAsPercentageOfGrossOfferingProceeds" xlink:label="loc_galt_CommissionsAsPercentageOfGrossOfferingProceeds_308" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_267" xlink:to="loc_galt_CommissionsAsPercentageOfGrossOfferingProceeds_308" use="optional" order="6" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_CommonSharesIssuableUponConversionOfConvertibleStock" xlink:label="loc_galt_CommonSharesIssuableUponConversionOfConvertibleStock_344" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_267" xlink:to="loc_galt_CommonSharesIssuableUponConversionOfConvertibleStock_344" use="optional" order="7" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_CommonStockMarketPrice" xlink:label="loc_galt_CommonStockMarketPrice_357" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_267" xlink:to="loc_galt_CommonStockMarketPrice_357" use="optional" order="8" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_CommonStockSharesAuthorized" xlink:label="loc_us-gaap_CommonStockSharesAuthorized_302" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_267" xlink:to="loc_us-gaap_CommonStockSharesAuthorized_302" use="optional" order="9" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_CommonStockSharesIssued" xlink:label="loc_us-gaap_CommonStockSharesIssued_321" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_267" xlink:to="loc_us-gaap_CommonStockSharesIssued_321" use="optional" order="10" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_CommonStockValue" xlink:label="loc_us-gaap_CommonStockValue_320" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_267" xlink:to="loc_us-gaap_CommonStockValue_320" use="optional" order="11" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_ConversionOfStockConversionPrice" xlink:label="loc_galt_ConversionOfStockConversionPrice_352" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_267" xlink:to="loc_galt_ConversionOfStockConversionPrice_352" use="optional" order="12" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ConversionOfStockSharesConverted1" xlink:label="loc_us-gaap_ConversionOfStockSharesConverted1_365" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_267" xlink:to="loc_us-gaap_ConversionOfStockSharesConverted1_365" use="optional" order="13" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ConversionOfStockSharesIssued1" xlink:label="loc_us-gaap_ConversionOfStockSharesIssued1_366" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_267" xlink:to="loc_us-gaap_ConversionOfStockSharesIssued1_366" use="optional" order="14" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ConvertiblePreferredStockSharesIssuedUponConversion" xlink:label="loc_us-gaap_ConvertiblePreferredStockSharesIssuedUponConversion_348" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_267" xlink:to="loc_us-gaap_ConvertiblePreferredStockSharesIssuedUponConversion_348" use="optional" order="15" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ConvertiblePreferredStockSharesReservedForFutureIssuance" xlink:label="loc_us-gaap_ConvertiblePreferredStockSharesReservedForFutureIssuance_346" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_267" xlink:to="loc_us-gaap_ConvertiblePreferredStockSharesReservedForFutureIssuance_346" use="optional" order="16" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_DividendAccrualRateOnInvestmentsInAffiliates" xlink:label="loc_galt_DividendAccrualRateOnInvestmentsInAffiliates_351" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_267" xlink:to="loc_galt_DividendAccrualRateOnInvestmentsInAffiliates_351" use="optional" order="17" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_DividendAsPercentageOfNetSales" xlink:label="loc_galt_DividendAsPercentageOfNetSales_367" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_267" xlink:to="loc_galt_DividendAsPercentageOfNetSales_367" use="optional" order="18" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_DividendPaymentDateDayAndMonth" xlink:label="loc_galt_DividendPaymentDateDayAndMonth_330" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_267" xlink:to="loc_galt_DividendPaymentDateDayAndMonth_330" use="optional" order="19" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_DividendsPreferredStock" xlink:label="loc_us-gaap_DividendsPreferredStock_368" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_267" xlink:to="loc_us-gaap_DividendsPreferredStock_368" use="optional" order="20" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_EquityIssuancePerShareAmount1" xlink:label="loc_galt_EquityIssuancePerShareAmount1_324" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_267" xlink:to="loc_galt_EquityIssuancePerShareAmount1_324" use="optional" order="21" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_NumberOfConsecutiveTradingDays" xlink:label="loc_galt_NumberOfConsecutiveTradingDays_329" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_267" xlink:to="loc_galt_NumberOfConsecutiveTradingDays_329" use="optional" order="22" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_NumberOfConsecutiveTradingDaysForConversionOfPreferredStock" xlink:label="loc_galt_NumberOfConsecutiveTradingDaysForConversionOfPreferredStock_333" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_267" xlink:to="loc_galt_NumberOfConsecutiveTradingDaysForConversionOfPreferredStock_333" use="optional" order="23" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_NumberOfTradingDays" xlink:label="loc_galt_NumberOfTradingDays_356" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_267" xlink:to="loc_galt_NumberOfTradingDays_356" use="optional" order="24" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_NumberOfWarrantsInEachUnit" xlink:label="loc_galt_NumberOfWarrantsInEachUnit_326" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_267" xlink:to="loc_galt_NumberOfWarrantsInEachUnit_326" use="optional" order="25" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_NumberOfWarrantsIssuedPursuantToTermsOfPurchaseAgreement" xlink:label="loc_galt_NumberOfWarrantsIssuedPursuantToTermsOfPurchaseAgreement_350" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_267" xlink:to="loc_galt_NumberOfWarrantsIssuedPursuantToTermsOfPurchaseAgreement_350" use="optional" order="26" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_PaymentsOfStockIssuanceCosts" xlink:label="loc_us-gaap_PaymentsOfStockIssuanceCosts_335" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_267" xlink:to="loc_us-gaap_PaymentsOfStockIssuanceCosts_335" use="optional" order="27" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_PercentageOfWeightedAverageStockPriceForDividendsPriorToPaymentDate" xlink:label="loc_galt_PercentageOfWeightedAverageStockPriceForDividendsPriorToPaymentDate_328" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_267" xlink:to="loc_galt_PercentageOfWeightedAverageStockPriceForDividendsPriorToPaymentDate_328" use="optional" order="28" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_PostConversionDividendRightsIssued" xlink:label="loc_galt_PostConversionDividendRightsIssued_369" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_267" xlink:to="loc_galt_PostConversionDividendRightsIssued_369" use="optional" order="29" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_PostConversionRedemptionRightReturnOnInvestmentMultiplierAmount" xlink:label="loc_galt_PostConversionRedemptionRightReturnOnInvestmentMultiplierAmount_371" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_267" xlink:to="loc_galt_PostConversionRedemptionRightReturnOnInvestmentMultiplierAmount_371" use="optional" order="30" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_PreferredStockDividendRatePercentage" xlink:label="loc_us-gaap_PreferredStockDividendRatePercentage_323" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_267" xlink:to="loc_us-gaap_PreferredStockDividendRatePercentage_323" use="optional" order="31" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_PreferredStockLiquidationPreference" xlink:label="loc_us-gaap_PreferredStockLiquidationPreference_353" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_267" xlink:to="loc_us-gaap_PreferredStockLiquidationPreference_353" use="optional" order="32" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_PreferredStockParOrStatedValuePerShare" xlink:label="loc_us-gaap_PreferredStockParOrStatedValuePerShare_362" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_267" xlink:to="loc_us-gaap_PreferredStockParOrStatedValuePerShare_362" use="optional" order="33" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_PreferredStockRedemptionPricePerShare" xlink:label="loc_us-gaap_PreferredStockRedemptionPricePerShare_372" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_267" xlink:to="loc_us-gaap_PreferredStockRedemptionPricePerShare_372" use="optional" order="34" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_PreferredStockSharesAuthorized" xlink:label="loc_us-gaap_PreferredStockSharesAuthorized_304" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_267" xlink:to="loc_us-gaap_PreferredStockSharesAuthorized_304" use="optional" order="35" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_PreferredStockStatedValuePerShare" xlink:label="loc_galt_PreferredStockStatedValuePerShare_363" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_267" xlink:to="loc_galt_PreferredStockStatedValuePerShare_363" use="optional" order="36" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ProceedsFromIssuanceOfCommonStock" xlink:label="loc_us-gaap_ProceedsFromIssuanceOfCommonStock_310" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_267" xlink:to="loc_us-gaap_ProceedsFromIssuanceOfCommonStock_310" use="optional" order="37" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ProceedsFromIssuanceOfPreferredStockAndPreferenceStock" xlink:label="loc_us-gaap_ProceedsFromIssuanceOfPreferredStockAndPreferenceStock_347" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_267" xlink:to="loc_us-gaap_ProceedsFromIssuanceOfPreferredStockAndPreferenceStock_347" use="optional" order="38" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_ProceedsFromIssuanceOfPreferredStockNetOfIssuanceCosts" xlink:label="loc_galt_ProceedsFromIssuanceOfPreferredStockNetOfIssuanceCosts_364" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_267" xlink:to="loc_galt_ProceedsFromIssuanceOfPreferredStockNetOfIssuanceCosts_364" use="optional" order="39" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ProceedsFromIssuanceOfPrivatePlacement" xlink:label="loc_us-gaap_ProceedsFromIssuanceOfPrivatePlacement_311" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_267" xlink:to="loc_us-gaap_ProceedsFromIssuanceOfPrivatePlacement_311" use="optional" order="40" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ProceedsFromIssuanceOrSaleOfEquity" xlink:label="loc_us-gaap_ProceedsFromIssuanceOrSaleOfEquity_336" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_267" xlink:to="loc_us-gaap_ProceedsFromIssuanceOrSaleOfEquity_336" use="optional" order="41" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_ProceedsFromSubscriptionAdvances" xlink:label="loc_galt_ProceedsFromSubscriptionAdvances_334" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_267" xlink:to="loc_galt_ProceedsFromSubscriptionAdvances_334" use="optional" order="42" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ProceedsFromWarrantExercises" xlink:label="loc_us-gaap_ProceedsFromWarrantExercises_358" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_267" xlink:to="loc_us-gaap_ProceedsFromWarrantExercises_358" use="optional" order="43" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_RedemptionPeriod" xlink:label="loc_galt_RedemptionPeriod_370" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_267" xlink:to="loc_galt_RedemptionPeriod_370" use="optional" order="44" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ResearchAndDevelopmentExpense" xlink:label="loc_us-gaap_ResearchAndDevelopmentExpense_322" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_267" xlink:to="loc_us-gaap_ResearchAndDevelopmentExpense_322" use="optional" order="45" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ScheduleOfStockByClassTable" xlink:label="loc_us-gaap_ScheduleOfStockByClassTable_279" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ClassOfStockLineItems_267" xlink:to="loc_us-gaap_ScheduleOfStockByClassTable_279" xbrldt:contextElement="segment" xbrldt:closed="true" use="optional" order="46" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ClassOfWarrantOrRightAxis" xlink:label="loc_us-gaap_ClassOfWarrantOrRightAxis_268" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_279" xlink:to="loc_us-gaap_ClassOfWarrantOrRightAxis_268" use="optional" order="47" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ClassOfWarrantOrRightDomain" xlink:label="loc_us-gaap_ClassOfWarrantOrRightDomain_269" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ClassOfWarrantOrRightAxis_268" xlink:to="loc_us-gaap_ClassOfWarrantOrRightDomain_269" use="optional" order="48" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ClassOfWarrantOrRightDomain" xlink:label="loc_us-gaap_ClassOfWarrantOrRightDomain_269_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ClassOfWarrantOrRightAxis_268" xlink:to="loc_us-gaap_ClassOfWarrantOrRightDomain_269_default" use="optional" order="49" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_SeriesAAndBWarrantsMember" xlink:label="loc_galt_SeriesAAndBWarrantsMember_375" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfWarrantOrRightDomain_269" xlink:to="loc_galt_SeriesAAndBWarrantsMember_375" use="optional" order="50" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_SeriesAOneWarrantMember" xlink:label="loc_galt_SeriesAOneWarrantMember_376" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfWarrantOrRightDomain_269" xlink:to="loc_galt_SeriesAOneWarrantMember_376" use="optional" order="51" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_SeriesATwoWarrantMember" xlink:label="loc_galt_SeriesATwoWarrantMember_377" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfWarrantOrRightDomain_269" xlink:to="loc_galt_SeriesATwoWarrantMember_377" use="optional" order="52" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_EquityInterestIssuedOrIssuableByTypeAxis" xlink:label="loc_us-gaap_EquityInterestIssuedOrIssuableByTypeAxis_271" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_279" xlink:to="loc_us-gaap_EquityInterestIssuedOrIssuableByTypeAxis_271" use="optional" order="53" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_EquityInterestIssuedOrIssuableTypeDomain" xlink:label="loc_us-gaap_EquityInterestIssuedOrIssuableTypeDomain_272" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_EquityInterestIssuedOrIssuableByTypeAxis_271" xlink:to="loc_us-gaap_EquityInterestIssuedOrIssuableTypeDomain_272" use="optional" order="54" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_EquityInterestIssuedOrIssuableTypeDomain" xlink:label="loc_us-gaap_EquityInterestIssuedOrIssuableTypeDomain_272_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_EquityInterestIssuedOrIssuableByTypeAxis_271" xlink:to="loc_us-gaap_EquityInterestIssuedOrIssuableTypeDomain_272_default" use="optional" order="55" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_PrivatePlacementTwentySeventeenMember" xlink:label="loc_galt_PrivatePlacementTwentySeventeenMember_382" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityInterestIssuedOrIssuableTypeDomain_272" xlink:to="loc_galt_PrivatePlacementTwentySeventeenMember_382" use="optional" order="56" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_LegalEntityAxis" xlink:label="loc_dei_LegalEntityAxis_273" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_279" xlink:to="loc_dei_LegalEntityAxis_273" use="optional" order="57" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_383" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_273" xlink:to="loc_dei_EntityDomain_383" use="optional" order="58" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_383_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_273" xlink:to="loc_dei_EntityDomain_383_default" use="optional" order="59" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_RangeAxis" xlink:label="loc_srt_RangeAxis_274" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_279" xlink:to="loc_srt_RangeAxis_274" use="optional" order="60" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_RangeMember" xlink:label="loc_srt_RangeMember_275" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_274" xlink:to="loc_srt_RangeMember_275" use="optional" order="61" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_RangeMember" xlink:label="loc_srt_RangeMember_275_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_274" xlink:to="loc_srt_RangeMember_275_default" use="optional" order="62" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_MaximumMember" xlink:label="loc_srt_MaximumMember_384" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_275" xlink:to="loc_srt_MaximumMember_384" use="optional" order="63" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_MinimumMember" xlink:label="loc_srt_MinimumMember_385" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_275" xlink:to="loc_srt_MinimumMember_385" use="optional" order="64" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_RelatedPartyTransactionsByRelatedPartyAxis" xlink:label="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_277" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_279" xlink:to="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_277" use="optional" order="65" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_RelatedPartyDomain" xlink:label="loc_us-gaap_RelatedPartyDomain_276" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_277" xlink:to="loc_us-gaap_RelatedPartyDomain_276" use="optional" order="66" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_RelatedPartyDomain" xlink:label="loc_us-gaap_RelatedPartyDomain_276_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_277" xlink:to="loc_us-gaap_RelatedPartyDomain_276_default" use="optional" order="67" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_VendorMember" xlink:label="loc_galt_VendorMember_386" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyDomain_276" xlink:to="loc_galt_VendorMember_386" use="optional" order="68" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementClassOfStockAxis" xlink:label="loc_us-gaap_StatementClassOfStockAxis_280" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_279" xlink:to="loc_us-gaap_StatementClassOfStockAxis_280" use="optional" order="69" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ClassOfStockDomain" xlink:label="loc_us-gaap_ClassOfStockDomain_266" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_280" xlink:to="loc_us-gaap_ClassOfStockDomain_266" use="optional" order="70" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ClassOfStockDomain" xlink:label="loc_us-gaap_ClassOfStockDomain_266_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_280" xlink:to="loc_us-gaap_ClassOfStockDomain_266_default" use="optional" order="71" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_CommonStockMember" xlink:label="loc_us-gaap_CommonStockMember_301" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_266" xlink:to="loc_us-gaap_CommonStockMember_301" use="optional" order="72" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SeriesAPreferredStockMember" xlink:label="loc_us-gaap_SeriesAPreferredStockMember_296" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_266" xlink:to="loc_us-gaap_SeriesAPreferredStockMember_296" use="optional" order="73" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_SeriesBOneConvertiblePreferredStockMember" xlink:label="loc_galt_SeriesBOneConvertiblePreferredStockMember_297" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_266" xlink:to="loc_galt_SeriesBOneConvertiblePreferredStockMember_297" use="optional" order="74" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_SeriesBOnePreferredStockMember" xlink:label="loc_galt_SeriesBOnePreferredStockMember_292" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_266" xlink:to="loc_galt_SeriesBOnePreferredStockMember_292" use="optional" order="75" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SeriesBPreferredStockMember" xlink:label="loc_us-gaap_SeriesBPreferredStockMember_299" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_266" xlink:to="loc_us-gaap_SeriesBPreferredStockMember_299" use="optional" order="76" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_SeriesBThreeConvertiblePreferredStockMember" xlink:label="loc_galt_SeriesBThreeConvertiblePreferredStockMember_291" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_266" xlink:to="loc_galt_SeriesBThreeConvertiblePreferredStockMember_291" use="optional" order="77" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_SeriesBThreePreferredStockMember" xlink:label="loc_galt_SeriesBThreePreferredStockMember_295" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_266" xlink:to="loc_galt_SeriesBThreePreferredStockMember_295" use="optional" order="78" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_SeriesBTwoConvertiblePreferredStockMember" xlink:label="loc_galt_SeriesBTwoConvertiblePreferredStockMember_298" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_266" xlink:to="loc_galt_SeriesBTwoConvertiblePreferredStockMember_298" use="optional" order="79" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_SeriesBTwoPreferredStockMember" xlink:label="loc_galt_SeriesBTwoPreferredStockMember_293" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_266" xlink:to="loc_galt_SeriesBTwoPreferredStockMember_293" use="optional" order="80" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SeriesCPreferredStockMember" xlink:label="loc_us-gaap_SeriesCPreferredStockMember_294" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_266" xlink:to="loc_us-gaap_SeriesCPreferredStockMember_294" use="optional" order="81" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementEquityComponentsAxis" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_281" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_279" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_281" use="optional" order="82" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_EquityComponentDomain" xlink:label="loc_us-gaap_EquityComponentDomain_270" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_281" xlink:to="loc_us-gaap_EquityComponentDomain_270" use="optional" order="83" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_EquityComponentDomain" xlink:label="loc_us-gaap_EquityComponentDomain_270_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_281" xlink:to="loc_us-gaap_EquityComponentDomain_270_default" use="optional" order="84" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_AdditionalPaidInCapitalMember" xlink:label="loc_us-gaap_AdditionalPaidInCapitalMember_378" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_270" xlink:to="loc_us-gaap_AdditionalPaidInCapitalMember_378" use="optional" order="85" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_BeneficialConversionFeatureMember" xlink:label="loc_galt_BeneficialConversionFeatureMember_380" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_270" xlink:to="loc_galt_BeneficialConversionFeatureMember_380" use="optional" order="86" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_WarrantMember" xlink:label="loc_us-gaap_WarrantMember_379" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_270" xlink:to="loc_us-gaap_WarrantMember_379" use="optional" order="87" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementScenarioAxis" xlink:label="loc_us-gaap_StatementScenarioAxis_282" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_279" xlink:to="loc_us-gaap_StatementScenarioAxis_282" use="optional" order="88" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="loc_us-gaap_ScenarioUnspecifiedDomain_278" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementScenarioAxis_282" xlink:to="loc_us-gaap_ScenarioUnspecifiedDomain_278" use="optional" order="89" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="loc_us-gaap_ScenarioUnspecifiedDomain_278_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementScenarioAxis_282" xlink:to="loc_us-gaap_ScenarioUnspecifiedDomain_278_default" use="optional" order="90" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_DividendPaymentOneMember" xlink:label="loc_galt_DividendPaymentOneMember_393" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScenarioUnspecifiedDomain_278" xlink:to="loc_galt_DividendPaymentOneMember_393" use="optional" order="91" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_DividendPaymentTwoMember" xlink:label="loc_galt_DividendPaymentTwoMember_394" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScenarioUnspecifiedDomain_278" xlink:to="loc_galt_DividendPaymentTwoMember_394" use="optional" order="92" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_FirstPaymentMember" xlink:label="loc_galt_FirstPaymentMember_389" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScenarioUnspecifiedDomain_278" xlink:to="loc_galt_FirstPaymentMember_389" use="optional" order="93" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_ScenarioOneMember" xlink:label="loc_galt_ScenarioOneMember_387" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScenarioUnspecifiedDomain_278" xlink:to="loc_galt_ScenarioOneMember_387" use="optional" order="94" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_ScenarioTwoMember" xlink:label="loc_galt_ScenarioTwoMember_388" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScenarioUnspecifiedDomain_278" xlink:to="loc_galt_ScenarioTwoMember_388" use="optional" order="95" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_SecondPaymentMember" xlink:label="loc_galt_SecondPaymentMember_390" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScenarioUnspecifiedDomain_278" xlink:to="loc_galt_SecondPaymentMember_390" use="optional" order="96" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_TitleOfIndividualAxis" xlink:label="loc_us-gaap_TitleOfIndividualAxis_283" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_279" xlink:to="loc_us-gaap_TitleOfIndividualAxis_283" use="optional" order="97" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_TitleOfIndividualWithRelationshipToEntityDomain" xlink:label="loc_us-gaap_TitleOfIndividualWithRelationshipToEntityDomain_284" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_TitleOfIndividualAxis_283" xlink:to="loc_us-gaap_TitleOfIndividualWithRelationshipToEntityDomain_284" use="optional" order="98" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_TitleOfIndividualWithRelationshipToEntityDomain" xlink:label="loc_us-gaap_TitleOfIndividualWithRelationshipToEntityDomain_284_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_TitleOfIndividualAxis_283" xlink:to="loc_us-gaap_TitleOfIndividualWithRelationshipToEntityDomain_284_default" use="optional" order="99" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_TypeOfArrangementAxis" xlink:label="loc_us-gaap_TypeOfArrangementAxis_285" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_279" xlink:to="loc_us-gaap_TypeOfArrangementAxis_285" use="optional" order="100" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ArrangementsAndNonarrangementTransactionsMember" xlink:label="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_265" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_TypeOfArrangementAxis_285" xlink:to="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_265" use="optional" order="101" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ArrangementsAndNonarrangementTransactionsMember" xlink:label="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_265_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_TypeOfArrangementAxis_285" xlink:to="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_265_default" use="optional" order="102" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_AmendedBThreeSecuritiesPurchaseAgreementMember" xlink:label="loc_galt_AmendedBThreeSecuritiesPurchaseAgreementMember_290" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_265" xlink:to="loc_galt_AmendedBThreeSecuritiesPurchaseAgreementMember_290" use="optional" order="103" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_BThreeSecuritiesPurchaseAgreementMember" xlink:label="loc_galt_BThreeSecuritiesPurchaseAgreementMember_288" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_265" xlink:to="loc_galt_BThreeSecuritiesPurchaseAgreementMember_288" use="optional" order="104" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_OtherAgreementsMember" xlink:label="loc_galt_OtherAgreementsMember_287" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_265" xlink:to="loc_galt_OtherAgreementsMember_287" use="optional" order="105" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_TwoThousandFourteenAtMarketSalesAgreementMember" xlink:label="loc_galt_TwoThousandFourteenAtMarketSalesAgreementMember_286" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_265" xlink:to="loc_galt_TwoThousandFourteenAtMarketSalesAgreementMember_286" use="optional" order="106" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_TwoThousandSeventeenAtMarketSalesAgreementMember" xlink:label="loc_galt_TwoThousandSeventeenAtMarketSalesAgreementMember_289" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_265" xlink:to="loc_galt_TwoThousandSeventeenAtMarketSalesAgreementMember_289" use="optional" order="107" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_MeasurementInputTypeAxis" xlink:label="loc_us-gaap_MeasurementInputTypeAxis_322103" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_279" xlink:to="loc_us-gaap_MeasurementInputTypeAxis_322103" use="optional" order="108" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_MeasurementInputTypeDomain" xlink:label="loc_us-gaap_MeasurementInputTypeDomain_322104" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_MeasurementInputTypeAxis_322103" xlink:to="loc_us-gaap_MeasurementInputTypeDomain_322104" use="optional" order="109" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_MeasurementInputTypeDomain" xlink:label="loc_us-gaap_MeasurementInputTypeDomain_322104_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_MeasurementInputTypeAxis_322103" xlink:to="loc_us-gaap_MeasurementInputTypeDomain_322104_default" use="optional" order="110" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_MeasurementInputPriceVolatilityMember" xlink:label="loc_us-gaap_MeasurementInputPriceVolatilityMember_322105" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MeasurementInputTypeDomain_322104" xlink:to="loc_us-gaap_MeasurementInputPriceVolatilityMember_322105" use="optional" order="111" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_MeasurementInputExpectedTermMember" xlink:label="loc_us-gaap_MeasurementInputExpectedTermMember_322106" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MeasurementInputTypeDomain_322104" xlink:to="loc_us-gaap_MeasurementInputExpectedTermMember_322106" use="optional" order="112" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_MeasurementInputRiskFreeInterestRateMember" xlink:label="loc_us-gaap_MeasurementInputRiskFreeInterestRateMember_322107" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MeasurementInputTypeDomain_322104" xlink:to="loc_us-gaap_MeasurementInputRiskFreeInterestRateMember_322107" use="optional" order="113" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_MeasurementInputExpectedDividendRateMember" xlink:label="loc_us-gaap_MeasurementInputExpectedDividendRateMember_322108" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MeasurementInputTypeDomain_322104" xlink:to="loc_us-gaap_MeasurementInputExpectedDividendRateMember_322108" use="optional" order="114" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsExercised" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsExercised_359" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_267" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsExercised_359" use="optional" order="115" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_ShareConversionRatio" xlink:label="loc_galt_ShareConversionRatio_331" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_267" xlink:to="loc_galt_ShareConversionRatio_331" use="optional" order="116" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SharePrice" xlink:label="loc_us-gaap_SharePrice_315" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_267" xlink:to="loc_us-gaap_SharePrice_315" use="optional" order="117" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StockAndWarrantsIssuedDuringPeriodValuePreferredStockAndWarrants" xlink:label="loc_us-gaap_StockAndWarrantsIssuedDuringPeriodValuePreferredStockAndWarrants_314" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_267" xlink:to="loc_us-gaap_StockAndWarrantsIssuedDuringPeriodValuePreferredStockAndWarrants_314" use="optional" order="118" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_StockIssuableShares" xlink:label="loc_galt_StockIssuableShares_342" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_267" xlink:to="loc_galt_StockIssuableShares_342" use="optional" order="119" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_StockIssuableValue" xlink:label="loc_galt_StockIssuableValue_343" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_267" xlink:to="loc_galt_StockIssuableValue_343" use="optional" order="120" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StockIssuedDuringPeriodSharesConversionOfConvertibleSecurities" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesConversionOfConvertibleSecurities_341" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_267" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesConversionOfConvertibleSecurities_341" use="optional" order="121" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StockIssuedDuringPeriodSharesNewIssues" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesNewIssues_309" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_267" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesNewIssues_309" use="optional" order="122" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_TemporaryEquityAggregateAmountOfRedemptionRequirement" xlink:label="loc_us-gaap_TemporaryEquityAggregateAmountOfRedemptionRequirement_374" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_267" xlink:to="loc_us-gaap_TemporaryEquityAggregateAmountOfRedemptionRequirement_374" use="optional" order="123" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_TemporaryEquitySharesAuthorized" xlink:label="loc_us-gaap_TemporaryEquitySharesAuthorized_305" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_267" xlink:to="loc_us-gaap_TemporaryEquitySharesAuthorized_305" use="optional" order="124" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_TerminationNoticePeriod" xlink:label="loc_galt_TerminationNoticePeriod_355" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_267" xlink:to="loc_galt_TerminationNoticePeriod_355" use="optional" order="125" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_UndesignatedCapitalStockSharesAuthorized" xlink:label="loc_galt_UndesignatedCapitalStockSharesAuthorized_303" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_267" xlink:to="loc_galt_UndesignatedCapitalStockSharesAuthorized_303" use="optional" order="126" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_WarrantExpirationDate" xlink:label="loc_galt_WarrantExpirationDate_319" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_267" xlink:to="loc_galt_WarrantExpirationDate_319" use="optional" order="127" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_WarrantIssuedToPurchaseCommonStock" xlink:label="loc_galt_WarrantIssuedToPurchaseCommonStock_349" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_267" xlink:to="loc_galt_WarrantIssuedToPurchaseCommonStock_349" use="optional" order="128" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_WarrantsAndRightsOutstanding" xlink:label="loc_us-gaap_WarrantsAndRightsOutstanding_338" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_267" xlink:to="loc_us-gaap_WarrantsAndRightsOutstanding_338" use="optional" order="129" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_SharesRelatingToPriorConversionDividendRights" xlink:label="loc_galt_SharesRelatingToPriorConversionDividendRights_321067" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_267" xlink:to="loc_galt_SharesRelatingToPriorConversionDividendRights_321067" use="optional" order="130" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_DebtInstrumentMeasurementInput" xlink:label="loc_us-gaap_DebtInstrumentMeasurementInput_322067" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_267" xlink:to="loc_us-gaap_DebtInstrumentMeasurementInput_322067" use="optional" order="131" />
</definitionLink>
<definitionLink xlink:type="extended" xlink:role="http://www.galectintherapeutics.com/role/ReturnOnInvestmentDetail">
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ClassOfStockLineItems" xlink:label="loc_us-gaap_ClassOfStockLineItems_399" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_PercentageOfReturnOnInvestment" xlink:label="loc_galt_PercentageOfReturnOnInvestment_404" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_399" xlink:to="loc_galt_PercentageOfReturnOnInvestment_404" use="optional" order="1" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_RedemptionDateDescription" xlink:label="loc_galt_RedemptionDateDescription_405" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_399" xlink:to="loc_galt_RedemptionDateDescription_405" use="optional" order="2" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ScheduleOfStockByClassTable" xlink:label="loc_us-gaap_ScheduleOfStockByClassTable_403" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ClassOfStockLineItems_399" xlink:to="loc_us-gaap_ScheduleOfStockByClassTable_403" xbrldt:contextElement="segment" xbrldt:closed="true" use="optional" order="3" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_LegalEntityAxis" xlink:label="loc_dei_LegalEntityAxis_400" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_403" xlink:to="loc_dei_LegalEntityAxis_400" use="optional" order="4" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_406" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_400" xlink:to="loc_dei_EntityDomain_406" use="optional" order="5" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_406_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_400" xlink:to="loc_dei_EntityDomain_406_default" use="optional" order="6" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_PeriodAxis" xlink:label="loc_galt_PeriodAxis_401" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_403" xlink:to="loc_galt_PeriodAxis_401" use="optional" order="7" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_PeriodDomain" xlink:label="loc_galt_PeriodDomain_402" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_galt_PeriodAxis_401" xlink:to="loc_galt_PeriodDomain_402" use="optional" order="8" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_PeriodDomain" xlink:label="loc_galt_PeriodDomain_402_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_galt_PeriodAxis_401" xlink:to="loc_galt_PeriodDomain_402_default" use="optional" order="9" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_PeriodEightMember" xlink:label="loc_galt_PeriodEightMember_414" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_galt_PeriodDomain_402" xlink:to="loc_galt_PeriodEightMember_414" use="optional" order="10" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_PeriodFiveMember" xlink:label="loc_galt_PeriodFiveMember_411" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_galt_PeriodDomain_402" xlink:to="loc_galt_PeriodFiveMember_411" use="optional" order="11" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_PeriodFourMember" xlink:label="loc_galt_PeriodFourMember_410" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_galt_PeriodDomain_402" xlink:to="loc_galt_PeriodFourMember_410" use="optional" order="12" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_PeriodOneMember" xlink:label="loc_galt_PeriodOneMember_407" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_galt_PeriodDomain_402" xlink:to="loc_galt_PeriodOneMember_407" use="optional" order="13" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_PeriodSevenMember" xlink:label="loc_galt_PeriodSevenMember_413" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_galt_PeriodDomain_402" xlink:to="loc_galt_PeriodSevenMember_413" use="optional" order="14" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_PeriodSixMember" xlink:label="loc_galt_PeriodSixMember_412" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_galt_PeriodDomain_402" xlink:to="loc_galt_PeriodSixMember_412" use="optional" order="15" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_PeriodThreeMember" xlink:label="loc_galt_PeriodThreeMember_409" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_galt_PeriodDomain_402" xlink:to="loc_galt_PeriodThreeMember_409" use="optional" order="16" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_PeriodTwoMember" xlink:label="loc_galt_PeriodTwoMember_408" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_galt_PeriodDomain_402" xlink:to="loc_galt_PeriodTwoMember_408" use="optional" order="17" />
</definitionLink>
<definitionLink xlink:type="extended" xlink:role="http://www.galectintherapeutics.com/role/SummaryOfWarrantActivityDetail">
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_421" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_419" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_421" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_419" xbrldt:contextElement="segment" xbrldt:closed="true" use="optional" order="1" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_LegalEntityAxis" xlink:label="loc_dei_LegalEntityAxis_418" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_419" xlink:to="loc_dei_LegalEntityAxis_418" use="optional" order="2" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_424" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_418" xlink:to="loc_dei_EntityDomain_424" use="optional" order="3" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_424_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_418" xlink:to="loc_dei_EntityDomain_424_default" use="optional" order="4" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementEquityComponentsAxis" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_422" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_419" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_422" use="optional" order="5" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_EquityComponentDomain" xlink:label="loc_us-gaap_EquityComponentDomain_417" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_422" xlink:to="loc_us-gaap_EquityComponentDomain_417" use="optional" order="6" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_EquityComponentDomain" xlink:label="loc_us-gaap_EquityComponentDomain_417_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_422" xlink:to="loc_us-gaap_EquityComponentDomain_417_default" use="optional" order="7" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_WarrantMember" xlink:label="loc_us-gaap_WarrantMember_423" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_417" xlink:to="loc_us-gaap_WarrantMember_423" use="optional" order="8" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_420" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_421" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_420" use="optional" order="9" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod_427" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_420" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod_427" use="optional" order="10" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_426" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_420" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_426" use="optional" order="11" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_425" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_420" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_425" use="optional" order="12" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsExercised" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsExercised_428" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_420" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsExercised_428" use="optional" order="13" />
</definitionLink>
<definitionLink xlink:type="extended" xlink:role="http://www.galectintherapeutics.com/role/SummaryOfInformationWithRegardToOutstandingWarrantsIssuedInConnectionWithEquityAndDebtFinancingAndConsultantsDetail">
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_EquityLineItems" xlink:label="loc_galt_EquityLineItems_434" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ClassOfWarrantOrRighstDateFromWhichWarrantsOrRightsExercisable" xlink:label="loc_us-gaap_ClassOfWarrantOrRighstDateFromWhichWarrantsOrRightsExercisable_461" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_galt_EquityLineItems_434" xlink:to="loc_us-gaap_ClassOfWarrantOrRighstDateFromWhichWarrantsOrRightsExercisable_461" use="optional" order="1" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1" xlink:label="loc_us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1_460" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_galt_EquityLineItems_434" xlink:to="loc_us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1_460" use="optional" order="2" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ClassOfWarrantOrRightOutstanding" xlink:label="loc_us-gaap_ClassOfWarrantOrRightOutstanding_459" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_galt_EquityLineItems_434" xlink:to="loc_us-gaap_ClassOfWarrantOrRightOutstanding_459" use="optional" order="3" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_ClassOfWarrantsOrRightsDateFromWhichWarrantsOrRightsExercisableDescription" xlink:label="loc_galt_ClassOfWarrantsOrRightsDateFromWhichWarrantsOrRightsExercisableDescription_457" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_galt_EquityLineItems_434" xlink:to="loc_galt_ClassOfWarrantsOrRightsDateFromWhichWarrantsOrRightsExercisableDescription_457" use="optional" order="4" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_EquityTable" xlink:label="loc_galt_EquityTable_435" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_galt_EquityLineItems_434" xlink:to="loc_galt_EquityTable_435" xbrldt:contextElement="segment" xbrldt:closed="true" use="optional" order="5" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_EquityInterestIssuedOrIssuableByTypeAxis" xlink:label="loc_us-gaap_EquityInterestIssuedOrIssuableByTypeAxis_432" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_galt_EquityTable_435" xlink:to="loc_us-gaap_EquityInterestIssuedOrIssuableByTypeAxis_432" use="optional" order="6" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_EquityInterestIssuedOrIssuableTypeDomain" xlink:label="loc_us-gaap_EquityInterestIssuedOrIssuableTypeDomain_433" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_EquityInterestIssuedOrIssuableByTypeAxis_432" xlink:to="loc_us-gaap_EquityInterestIssuedOrIssuableTypeDomain_433" use="optional" order="7" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_EquityInterestIssuedOrIssuableTypeDomain" xlink:label="loc_us-gaap_EquityInterestIssuedOrIssuableTypeDomain_433_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_EquityInterestIssuedOrIssuableByTypeAxis_432" xlink:to="loc_us-gaap_EquityInterestIssuedOrIssuableTypeDomain_433_default" use="optional" order="8" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_AprilThirtyTwentyTenSeriesBTwoTransactionMember" xlink:label="loc_galt_AprilThirtyTwentyTenSeriesBTwoTransactionMember_446" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityInterestIssuedOrIssuableTypeDomain_433" xlink:to="loc_galt_AprilThirtyTwentyTenSeriesBTwoTransactionMember_446" use="optional" order="9" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_AugustTwelveTwentyZeroNineSeriesBTwoTransactionMember" xlink:label="loc_galt_AugustTwelveTwentyZeroNineSeriesBTwoTransactionMember_440" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityInterestIssuedOrIssuableTypeDomain_433" xlink:to="loc_galt_AugustTwelveTwentyZeroNineSeriesBTwoTransactionMember_440" use="optional" order="10" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_DecemberEightTwentyZeroNineSeriesBTwoTransactionMember" xlink:label="loc_galt_DecemberEightTwentyZeroNineSeriesBTwoTransactionMember_443" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityInterestIssuedOrIssuableTypeDomain_433" xlink:to="loc_galt_DecemberEightTwentyZeroNineSeriesBTwoTransactionMember_443" use="optional" order="11" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_DecemberNineteenTwentySeventeenLineOfCreditWarrantsMember" xlink:label="loc_galt_DecemberNineteenTwentySeventeenLineOfCreditWarrantsMember_456" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityInterestIssuedOrIssuableTypeDomain_433" xlink:to="loc_galt_DecemberNineteenTwentySeventeenLineOfCreditWarrantsMember_456" use="optional" order="12" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_DecemberTwentyEightTwentySixteenPrivatePlacementMember" xlink:label="loc_galt_DecemberTwentyEightTwentySixteenPrivatePlacementMember_453" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityInterestIssuedOrIssuableTypeDomain_433" xlink:to="loc_galt_DecemberTwentyEightTwentySixteenPrivatePlacementMember_453" use="optional" order="13" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_DecemberTwentyThreeTwentySixteenSeriesBThreeTransactionMember" xlink:label="loc_galt_DecemberTwentyThreeTwentySixteenSeriesBThreeTransactionMember_452" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityInterestIssuedOrIssuableTypeDomain_433" xlink:to="loc_galt_DecemberTwentyThreeTwentySixteenSeriesBThreeTransactionMember_452" use="optional" order="14" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_DecemberTwentyTwoTwentySixteenPrivatePlacementMember" xlink:label="loc_galt_DecemberTwentyTwoTwentySixteenPrivatePlacementMember_451" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityInterestIssuedOrIssuableTypeDomain_433" xlink:to="loc_galt_DecemberTwentyTwoTwentySixteenPrivatePlacementMember_451" use="optional" order="15" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_FebruaryTwelveTwentyZeroNineSeriesBOneTransactionMember" xlink:label="loc_galt_FebruaryTwelveTwentyZeroNineSeriesBOneTransactionMember_437" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityInterestIssuedOrIssuableTypeDomain_433" xlink:to="loc_galt_FebruaryTwelveTwentyZeroNineSeriesBOneTransactionMember_437" use="optional" order="16" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_FebruaryTwentySevenTwentySeventeenPrivatePlacementMember" xlink:label="loc_galt_FebruaryTwentySevenTwentySeventeenPrivatePlacementMember_454" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityInterestIssuedOrIssuableTypeDomain_433" xlink:to="loc_galt_FebruaryTwentySevenTwentySeventeenPrivatePlacementMember_454" use="optional" order="17" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_JanuaryTwentyNineTwentyTenSeriesBTwoTransactionMember" xlink:label="loc_galt_JanuaryTwentyNineTwentyTenSeriesBTwoTransactionMember_444" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityInterestIssuedOrIssuableTypeDomain_433" xlink:to="loc_galt_JanuaryTwentyNineTwentyTenSeriesBTwoTransactionMember_444" use="optional" order="18" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_JuneThirtyTwentyZeroNineSeriesBTwoTransactionMember" xlink:label="loc_galt_JuneThirtyTwentyZeroNineSeriesBTwoTransactionMember_439" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityInterestIssuedOrIssuableTypeDomain_433" xlink:to="loc_galt_JuneThirtyTwentyZeroNineSeriesBTwoTransactionMember_439" use="optional" order="19" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_MarchEightTwentyTenSeriesBTwoTransactionMember" xlink:label="loc_galt_MarchEightTwentyTenSeriesBTwoTransactionMember_445" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityInterestIssuedOrIssuableTypeDomain_433" xlink:to="loc_galt_MarchEightTwentyTenSeriesBTwoTransactionMember_445" use="optional" order="20" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_MayTenTwentyTenSeriesBTwoTransactionMember" xlink:label="loc_galt_MayTenTwentyTenSeriesBTwoTransactionMember_447" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityInterestIssuedOrIssuableTypeDomain_433" xlink:to="loc_galt_MayTenTwentyTenSeriesBTwoTransactionMember_447" use="optional" order="21" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_MayThirteenTwentyZeroNineSeriesBTwoTransactionMember" xlink:label="loc_galt_MayThirteenTwentyZeroNineSeriesBTwoTransactionMember_438" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityInterestIssuedOrIssuableTypeDomain_433" xlink:to="loc_galt_MayThirteenTwentyZeroNineSeriesBTwoTransactionMember_438" use="optional" order="22" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_NovemberFourTwentyZeroNineSeriesBTwoTransactionMember" xlink:label="loc_galt_NovemberFourTwentyZeroNineSeriesBTwoTransactionMember_442" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityInterestIssuedOrIssuableTypeDomain_433" xlink:to="loc_galt_NovemberFourTwentyZeroNineSeriesBTwoTransactionMember_442" use="optional" order="23" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_NovemberTwentyFiveTwentyFifteenOfferingWarrantsMember" xlink:label="loc_galt_NovemberTwentyFiveTwentyFifteenOfferingWarrantsMember_448" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityInterestIssuedOrIssuableTypeDomain_433" xlink:to="loc_galt_NovemberTwentyFiveTwentyFifteenOfferingWarrantsMember_448" use="optional" order="24" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_SeptemberThirtyTwentyZeroNineSeriesBTwoTransactionMember" xlink:label="loc_galt_SeptemberThirtyTwentyZeroNineSeriesBTwoTransactionMember_441" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityInterestIssuedOrIssuableTypeDomain_433" xlink:to="loc_galt_SeptemberThirtyTwentyZeroNineSeriesBTwoTransactionMember_441" use="optional" order="25" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_SeptemberTwentyNineTwentySixteenSeriesBThreeTransactionMember" xlink:label="loc_galt_SeptemberTwentyNineTwentySixteenSeriesBThreeTransactionMember_450" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityInterestIssuedOrIssuableTypeDomain_433" xlink:to="loc_galt_SeptemberTwentyNineTwentySixteenSeriesBThreeTransactionMember_450" use="optional" order="26" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_SeptemberTwentyTwoTwentySixteenSeriesBThreeTransactionMember" xlink:label="loc_galt_SeptemberTwentyTwoTwentySixteenSeriesBThreeTransactionMember_449" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityInterestIssuedOrIssuableTypeDomain_433" xlink:to="loc_galt_SeptemberTwentyTwoTwentySixteenSeriesBThreeTransactionMember_449" use="optional" order="27" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_TwentyEighteenAndTwentySeventeenWarrantsIssuedForServicesMember" xlink:label="loc_galt_TwentyEighteenAndTwentySeventeenWarrantsIssuedForServicesMember_455" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityInterestIssuedOrIssuableTypeDomain_433" xlink:to="loc_galt_TwentyEighteenAndTwentySeventeenWarrantsIssuedForServicesMember_455" use="optional" order="28" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_LegalEntityAxis" xlink:label="loc_dei_LegalEntityAxis_436" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_galt_EquityTable_435" xlink:to="loc_dei_LegalEntityAxis_436" use="optional" order="29" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_463" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_436" xlink:to="loc_dei_EntityDomain_463" use="optional" order="30" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_463_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_436" xlink:to="loc_dei_EntityDomain_463_default" use="optional" order="31" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_WarrantExpirationDate" xlink:label="loc_galt_WarrantExpirationDate_462" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_galt_EquityLineItems_434" xlink:to="loc_galt_WarrantExpirationDate_462" use="optional" order="32" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_WarrantsExpirationDateDescription" xlink:label="loc_galt_WarrantsExpirationDateDescription_458" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_galt_EquityLineItems_434" xlink:to="loc_galt_WarrantsExpirationDateDescription_458" use="optional" order="33" />
</definitionLink>
<definitionLink xlink:type="extended" xlink:role="http://www.galectintherapeutics.com/role/SummaryOfInformationWithRegardToOutstandingWarrantsIssuedInConnectionWithEquityAndDebtFinancingAndConsultantsParentheticalDetail">
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_EquityLineItems" xlink:label="loc_galt_EquityLineItems_468" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_EquityTable" xlink:label="loc_galt_EquityTable_469" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_galt_EquityLineItems_468" xlink:to="loc_galt_EquityTable_469" xbrldt:contextElement="segment" xbrldt:closed="true" use="optional" order="1" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_EquityInterestIssuedOrIssuableByTypeAxis" xlink:label="loc_us-gaap_EquityInterestIssuedOrIssuableByTypeAxis_466" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_galt_EquityTable_469" xlink:to="loc_us-gaap_EquityInterestIssuedOrIssuableByTypeAxis_466" use="optional" order="2" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_EquityInterestIssuedOrIssuableTypeDomain" xlink:label="loc_us-gaap_EquityInterestIssuedOrIssuableTypeDomain_467" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_EquityInterestIssuedOrIssuableByTypeAxis_466" xlink:to="loc_us-gaap_EquityInterestIssuedOrIssuableTypeDomain_467" use="optional" order="3" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_EquityInterestIssuedOrIssuableTypeDomain" xlink:label="loc_us-gaap_EquityInterestIssuedOrIssuableTypeDomain_467_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_EquityInterestIssuedOrIssuableByTypeAxis_466" xlink:to="loc_us-gaap_EquityInterestIssuedOrIssuableTypeDomain_467_default" use="optional" order="4" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_AprilThirtyTwentyTenSeriesBTwoTransactionMember" xlink:label="loc_galt_AprilThirtyTwentyTenSeriesBTwoTransactionMember_480" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityInterestIssuedOrIssuableTypeDomain_467" xlink:to="loc_galt_AprilThirtyTwentyTenSeriesBTwoTransactionMember_480" use="optional" order="5" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_AugustTwelveTwentyZeroNineSeriesBTwoTransactionMember" xlink:label="loc_galt_AugustTwelveTwentyZeroNineSeriesBTwoTransactionMember_474" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityInterestIssuedOrIssuableTypeDomain_467" xlink:to="loc_galt_AugustTwelveTwentyZeroNineSeriesBTwoTransactionMember_474" use="optional" order="6" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_DecemberEightTwentyZeroNineSeriesBTwoTransactionMember" xlink:label="loc_galt_DecemberEightTwentyZeroNineSeriesBTwoTransactionMember_477" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityInterestIssuedOrIssuableTypeDomain_467" xlink:to="loc_galt_DecemberEightTwentyZeroNineSeriesBTwoTransactionMember_477" use="optional" order="7" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_DecemberNineteenTwentySeventeenLineOfCreditWarrantsMember" xlink:label="loc_galt_DecemberNineteenTwentySeventeenLineOfCreditWarrantsMember_489" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityInterestIssuedOrIssuableTypeDomain_467" xlink:to="loc_galt_DecemberNineteenTwentySeventeenLineOfCreditWarrantsMember_489" use="optional" order="8" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_DecemberTwentyEightTwentySixteenPrivatePlacementMember" xlink:label="loc_galt_DecemberTwentyEightTwentySixteenPrivatePlacementMember_487" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityInterestIssuedOrIssuableTypeDomain_467" xlink:to="loc_galt_DecemberTwentyEightTwentySixteenPrivatePlacementMember_487" use="optional" order="9" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_DecemberTwentyThreeTwentySixteenSeriesBThreeTransactionMember" xlink:label="loc_galt_DecemberTwentyThreeTwentySixteenSeriesBThreeTransactionMember_486" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityInterestIssuedOrIssuableTypeDomain_467" xlink:to="loc_galt_DecemberTwentyThreeTwentySixteenSeriesBThreeTransactionMember_486" use="optional" order="10" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_DecemberTwentyTwoTwentySixteenPrivatePlacementMember" xlink:label="loc_galt_DecemberTwentyTwoTwentySixteenPrivatePlacementMember_485" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityInterestIssuedOrIssuableTypeDomain_467" xlink:to="loc_galt_DecemberTwentyTwoTwentySixteenPrivatePlacementMember_485" use="optional" order="11" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_FebruaryTwelveTwentyZeroNineSeriesBOneTransactionMember" xlink:label="loc_galt_FebruaryTwelveTwentyZeroNineSeriesBOneTransactionMember_471" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityInterestIssuedOrIssuableTypeDomain_467" xlink:to="loc_galt_FebruaryTwelveTwentyZeroNineSeriesBOneTransactionMember_471" use="optional" order="12" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_FebruaryTwentySevenTwentySeventeenPrivatePlacementMember" xlink:label="loc_galt_FebruaryTwentySevenTwentySeventeenPrivatePlacementMember_488" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityInterestIssuedOrIssuableTypeDomain_467" xlink:to="loc_galt_FebruaryTwentySevenTwentySeventeenPrivatePlacementMember_488" use="optional" order="13" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_JanuaryTwentyNineTwentyTenSeriesBTwoTransactionMember" xlink:label="loc_galt_JanuaryTwentyNineTwentyTenSeriesBTwoTransactionMember_478" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityInterestIssuedOrIssuableTypeDomain_467" xlink:to="loc_galt_JanuaryTwentyNineTwentyTenSeriesBTwoTransactionMember_478" use="optional" order="14" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_JuneThirtyTwentyZeroNineSeriesBTwoTransactionMember" xlink:label="loc_galt_JuneThirtyTwentyZeroNineSeriesBTwoTransactionMember_473" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityInterestIssuedOrIssuableTypeDomain_467" xlink:to="loc_galt_JuneThirtyTwentyZeroNineSeriesBTwoTransactionMember_473" use="optional" order="15" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_MarchEightTwentyTenSeriesBTwoTransactionMember" xlink:label="loc_galt_MarchEightTwentyTenSeriesBTwoTransactionMember_479" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityInterestIssuedOrIssuableTypeDomain_467" xlink:to="loc_galt_MarchEightTwentyTenSeriesBTwoTransactionMember_479" use="optional" order="16" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_MayTenTwentyTenSeriesBTwoTransactionMember" xlink:label="loc_galt_MayTenTwentyTenSeriesBTwoTransactionMember_481" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityInterestIssuedOrIssuableTypeDomain_467" xlink:to="loc_galt_MayTenTwentyTenSeriesBTwoTransactionMember_481" use="optional" order="17" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_MayThirteenTwentyZeroNineSeriesBTwoTransactionMember" xlink:label="loc_galt_MayThirteenTwentyZeroNineSeriesBTwoTransactionMember_472" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityInterestIssuedOrIssuableTypeDomain_467" xlink:to="loc_galt_MayThirteenTwentyZeroNineSeriesBTwoTransactionMember_472" use="optional" order="18" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_NovemberFourTwentyZeroNineSeriesBTwoTransactionMember" xlink:label="loc_galt_NovemberFourTwentyZeroNineSeriesBTwoTransactionMember_476" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityInterestIssuedOrIssuableTypeDomain_467" xlink:to="loc_galt_NovemberFourTwentyZeroNineSeriesBTwoTransactionMember_476" use="optional" order="19" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_NovemberTwentyFiveTwentyFifteenOfferingWarrantsMember" xlink:label="loc_galt_NovemberTwentyFiveTwentyFifteenOfferingWarrantsMember_482" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityInterestIssuedOrIssuableTypeDomain_467" xlink:to="loc_galt_NovemberTwentyFiveTwentyFifteenOfferingWarrantsMember_482" use="optional" order="20" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_SeptemberThirtyTwentyZeroNineSeriesBTwoTransactionMember" xlink:label="loc_galt_SeptemberThirtyTwentyZeroNineSeriesBTwoTransactionMember_475" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityInterestIssuedOrIssuableTypeDomain_467" xlink:to="loc_galt_SeptemberThirtyTwentyZeroNineSeriesBTwoTransactionMember_475" use="optional" order="21" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_SeptemberTwentyNineTwentySixteenSeriesBThreeTransactionMember" xlink:label="loc_galt_SeptemberTwentyNineTwentySixteenSeriesBThreeTransactionMember_484" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityInterestIssuedOrIssuableTypeDomain_467" xlink:to="loc_galt_SeptemberTwentyNineTwentySixteenSeriesBThreeTransactionMember_484" use="optional" order="22" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_SeptemberTwentyTwoTwentySixteenSeriesBThreeTransactionMember" xlink:label="loc_galt_SeptemberTwentyTwoTwentySixteenSeriesBThreeTransactionMember_483" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityInterestIssuedOrIssuableTypeDomain_467" xlink:to="loc_galt_SeptemberTwentyTwoTwentySixteenSeriesBThreeTransactionMember_483" use="optional" order="23" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_LegalEntityAxis" xlink:label="loc_dei_LegalEntityAxis_470" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_galt_EquityTable_469" xlink:to="loc_dei_LegalEntityAxis_470" use="optional" order="24" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_492" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_470" xlink:to="loc_dei_EntityDomain_492" use="optional" order="25" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_492_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_470" xlink:to="loc_dei_EntityDomain_492_default" use="optional" order="26" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_IssuanceOfWarrantsPricePerShare" xlink:label="loc_galt_IssuanceOfWarrantsPricePerShare_491" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_galt_EquityLineItems_468" xlink:to="loc_galt_IssuanceOfWarrantsPricePerShare_491" use="optional" order="27" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_WarrantIssueDate" xlink:label="loc_galt_WarrantIssueDate_490" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_galt_EquityLineItems_468" xlink:to="loc_galt_WarrantIssueDate_490" use="optional" order="28" />
</definitionLink>
<definitionLink xlink:type="extended" xlink:role="http://www.galectintherapeutics.com/role/WarrantsAdditionalInformationDetail">
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_499" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ClassOfWarrantOrRightOutstanding" xlink:label="loc_us-gaap_ClassOfWarrantOrRightOutstanding_513" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_499" xlink:to="loc_us-gaap_ClassOfWarrantOrRightOutstanding_513" use="optional" order="1" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_CommonStockWarrantsIssued" xlink:label="loc_galt_CommonStockWarrantsIssued_504" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_499" xlink:to="loc_galt_CommonStockWarrantsIssued_504" use="optional" order="2" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_IssuanceOfEquityWarrantsInConnectionWithEquityOfferings" xlink:label="loc_galt_IssuanceOfEquityWarrantsInConnectionWithEquityOfferings_508" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_499" xlink:to="loc_galt_IssuanceOfEquityWarrantsInConnectionWithEquityOfferings_508" use="optional" order="3" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_IssuanceOfWarrantsPricePerShare" xlink:label="loc_galt_IssuanceOfWarrantsPricePerShare_505" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_499" xlink:to="loc_galt_IssuanceOfWarrantsPricePerShare_505" use="optional" order="4" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ProceedsFromIssuanceOrSaleOfEquity" xlink:label="loc_us-gaap_ProceedsFromIssuanceOrSaleOfEquity_507" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_499" xlink:to="loc_us-gaap_ProceedsFromIssuanceOrSaleOfEquity_507" use="optional" order="5" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_ProceedsFromIssuanceOrSaleOfEquityGross" xlink:label="loc_galt_ProceedsFromIssuanceOrSaleOfEquityGross_506" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_499" xlink:to="loc_galt_ProceedsFromIssuanceOrSaleOfEquityGross_506" use="optional" order="6" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_498" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_499" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_498" xbrldt:contextElement="segment" xbrldt:closed="true" use="optional" order="7" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ClassOfWarrantOrRightAxis" xlink:label="loc_us-gaap_ClassOfWarrantOrRightAxis_495" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_498" xlink:to="loc_us-gaap_ClassOfWarrantOrRightAxis_495" use="optional" order="8" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ClassOfWarrantOrRightDomain" xlink:label="loc_us-gaap_ClassOfWarrantOrRightDomain_496" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ClassOfWarrantOrRightAxis_495" xlink:to="loc_us-gaap_ClassOfWarrantOrRightDomain_496" use="optional" order="9" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ClassOfWarrantOrRightDomain" xlink:label="loc_us-gaap_ClassOfWarrantOrRightDomain_496_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ClassOfWarrantOrRightAxis_495" xlink:to="loc_us-gaap_ClassOfWarrantOrRightDomain_496_default" use="optional" order="10" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_WarrantFourteenMember" xlink:label="loc_galt_WarrantFourteenMember_500" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfWarrantOrRightDomain_496" xlink:to="loc_galt_WarrantFourteenMember_500" use="optional" order="11" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_LegalEntityAxis" xlink:label="loc_dei_LegalEntityAxis_497" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_498" xlink:to="loc_dei_LegalEntityAxis_497" use="optional" order="12" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_501" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_497" xlink:to="loc_dei_EntityDomain_501" use="optional" order="13" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_501_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_497" xlink:to="loc_dei_EntityDomain_501_default" use="optional" order="14" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_MeasurementInputTypeAxis" xlink:label="loc_us-gaap_MeasurementInputTypeAxis_322134" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_498" xlink:to="loc_us-gaap_MeasurementInputTypeAxis_322134" use="optional" order="15" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_MeasurementInputTypeDomain" xlink:label="loc_us-gaap_MeasurementInputTypeDomain_322135" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_MeasurementInputTypeAxis_322134" xlink:to="loc_us-gaap_MeasurementInputTypeDomain_322135" use="optional" order="16" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_MeasurementInputTypeDomain" xlink:label="loc_us-gaap_MeasurementInputTypeDomain_322135_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_MeasurementInputTypeAxis_322134" xlink:to="loc_us-gaap_MeasurementInputTypeDomain_322135_default" use="optional" order="17" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_MeasurementInputExercisePriceMember" xlink:label="loc_us-gaap_MeasurementInputExercisePriceMember_322136" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MeasurementInputTypeDomain_322135" xlink:to="loc_us-gaap_MeasurementInputExercisePriceMember_322136" use="optional" order="18" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_MeasurementInputExpectedTermMember" xlink:label="loc_us-gaap_MeasurementInputExpectedTermMember_322140" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MeasurementInputTypeDomain_322135" xlink:to="loc_us-gaap_MeasurementInputExpectedTermMember_322140" use="optional" order="19" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_MeasurementInputPriceVolatilityMember" xlink:label="loc_us-gaap_MeasurementInputPriceVolatilityMember_322141" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MeasurementInputTypeDomain_322135" xlink:to="loc_us-gaap_MeasurementInputPriceVolatilityMember_322141" use="optional" order="20" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_MeasurementInputRiskFreeInterestRateMember" xlink:label="loc_us-gaap_MeasurementInputRiskFreeInterestRateMember_322142" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MeasurementInputTypeDomain_322135" xlink:to="loc_us-gaap_MeasurementInputRiskFreeInterestRateMember_322142" use="optional" order="21" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StockIssuedDuringPeriodSharesNewIssues" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesNewIssues_503" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_499" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesNewIssues_503" use="optional" order="22" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_UnitsIssuedDuringPeriodUnitsNewIssues" xlink:label="loc_galt_UnitsIssuedDuringPeriodUnitsNewIssues_502" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_499" xlink:to="loc_galt_UnitsIssuedDuringPeriodUnitsNewIssues_502" use="optional" order="23" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_WarrantsExpirationDate" xlink:label="loc_galt_WarrantsExpirationDate_514" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_499" xlink:to="loc_galt_WarrantsExpirationDate_514" use="optional" order="24" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_WarrantsAndRightsOutstandingMeasurementInput" xlink:label="loc_us-gaap_WarrantsAndRightsOutstandingMeasurementInput_322137" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_499" xlink:to="loc_us-gaap_WarrantsAndRightsOutstandingMeasurementInput_322137" use="optional" order="25" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_WarrantsAndRightsOutstandingTerm" xlink:label="loc_us-gaap_WarrantsAndRightsOutstandingTerm_322139" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_499" xlink:to="loc_us-gaap_WarrantsAndRightsOutstandingTerm_322139" use="optional" order="26" />
</definitionLink>
<definitionLink xlink:type="extended" xlink:role="http://www.galectintherapeutics.com/role/StockBasedCompensationAdditionalInformationDetail">
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_524" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_AllocatedShareBasedCompensationExpense" xlink:label="loc_us-gaap_AllocatedShareBasedCompensationExpense_552" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_524" xlink:to="loc_us-gaap_AllocatedShareBasedCompensationExpense_552" use="optional" order="1" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized_542" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_524" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized_542" use="optional" order="2" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_543" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_524" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_543" use="optional" order="3" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_523" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_524" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_523" xbrldt:contextElement="segment" xbrldt:closed="true" use="optional" order="4" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_AwardTypeAxis" xlink:label="loc_us-gaap_AwardTypeAxis_517" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_523" xlink:to="loc_us-gaap_AwardTypeAxis_517" use="optional" order="5" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_525" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_517" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_525" use="optional" order="6" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_525_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_517" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_525_default" use="optional" order="7" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_RestrictedStockMember" xlink:label="loc_us-gaap_RestrictedStockMember_553" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_525" xlink:to="loc_us-gaap_RestrictedStockMember_553" use="optional" order="8" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_LegalEntityAxis" xlink:label="loc_dei_LegalEntityAxis_518" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_523" xlink:to="loc_dei_LegalEntityAxis_518" use="optional" order="9" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_528" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_518" xlink:to="loc_dei_EntityDomain_528" use="optional" order="10" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_528_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_518" xlink:to="loc_dei_EntityDomain_528_default" use="optional" order="11" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_PlanNameAxis" xlink:label="loc_us-gaap_PlanNameAxis_519" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_523" xlink:to="loc_us-gaap_PlanNameAxis_519" use="optional" order="12" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_PlanNameDomain" xlink:label="loc_us-gaap_PlanNameDomain_520" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PlanNameAxis_519" xlink:to="loc_us-gaap_PlanNameDomain_520" use="optional" order="13" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_PlanNameDomain" xlink:label="loc_us-gaap_PlanNameDomain_520_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PlanNameAxis_519" xlink:to="loc_us-gaap_PlanNameDomain_520_default" use="optional" order="14" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_IncentiveCompensationPlanTwentyZeroNineMember" xlink:label="loc_galt_IncentiveCompensationPlanTwentyZeroNineMember_530" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanNameDomain_520" xlink:to="loc_galt_IncentiveCompensationPlanTwentyZeroNineMember_530" use="optional" order="15" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_NonQualifiedOptionsPlansMember" xlink:label="loc_galt_NonQualifiedOptionsPlansMember_529" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanNameDomain_520" xlink:to="loc_galt_NonQualifiedOptionsPlansMember_529" use="optional" order="16" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_RangeAxis" xlink:label="loc_srt_RangeAxis_521" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_523" xlink:to="loc_srt_RangeAxis_521" use="optional" order="17" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_RangeMember" xlink:label="loc_srt_RangeMember_522" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_521" xlink:to="loc_srt_RangeMember_522" use="optional" order="18" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_RangeMember" xlink:label="loc_srt_RangeMember_522_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_521" xlink:to="loc_srt_RangeMember_522_default" use="optional" order="19" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_MaximumMember" xlink:label="loc_srt_MaximumMember_532" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_522" xlink:to="loc_srt_MaximumMember_532" use="optional" order="20" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_MinimumMember" xlink:label="loc_srt_MinimumMember_531" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_522" xlink:to="loc_srt_MinimumMember_531" use="optional" order="21" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_TitleOfIndividualAxis" xlink:label="loc_us-gaap_TitleOfIndividualAxis_526" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_523" xlink:to="loc_us-gaap_TitleOfIndividualAxis_526" use="optional" order="22" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_TitleOfIndividualWithRelationshipToEntityDomain" xlink:label="loc_us-gaap_TitleOfIndividualWithRelationshipToEntityDomain_527" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_TitleOfIndividualAxis_526" xlink:to="loc_us-gaap_TitleOfIndividualWithRelationshipToEntityDomain_527" use="optional" order="23" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_TitleOfIndividualWithRelationshipToEntityDomain" xlink:label="loc_us-gaap_TitleOfIndividualWithRelationshipToEntityDomain_527_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_TitleOfIndividualAxis_526" xlink:to="loc_us-gaap_TitleOfIndividualWithRelationshipToEntityDomain_527_default" use="optional" order="24" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_NonEmployeeBoardOfDirectorsOneMember" xlink:label="loc_galt_NonEmployeeBoardOfDirectorsOneMember_555" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TitleOfIndividualWithRelationshipToEntityDomain_527" xlink:to="loc_galt_NonEmployeeBoardOfDirectorsOneMember_555" use="optional" order="25" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOptionsGrantsInPeriod" xlink:label="loc_galt_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOptionsGrantsInPeriod_535" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_524" xlink:to="loc_galt_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOptionsGrantsInPeriod_535" use="optional" order="26" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_547" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_524" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_547" use="optional" order="27" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate_538" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_524" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate_538" use="optional" order="28" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1_537" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_524" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1_537" use="optional" order="29" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized_533" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_524" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized_533" use="optional" order="30" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant_534" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_524" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant_534" use="optional" order="31" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_540" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_524" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_540" use="optional" order="32" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsNonvestedNumberOfShares" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsNonvestedNumberOfShares_541" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_524" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsNonvestedNumberOfShares_541" use="optional" order="33" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber_536" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_524" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber_536" use="optional" order="34" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedInPeriodFairValue1" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedInPeriodFairValue1_546" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_524" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedInPeriodFairValue1_546" use="optional" order="35" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedNumberOfShares" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedNumberOfShares_545" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_524" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedNumberOfShares_545" use="optional" order="36" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SharePrice" xlink:label="loc_us-gaap_SharePrice_539" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_524" xlink:to="loc_us-gaap_SharePrice_539" use="optional" order="37" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_544" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_524" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_544" use="optional" order="38" />
</definitionLink>
<definitionLink xlink:type="extended" xlink:role="http://www.galectintherapeutics.com/role/StockBasedCompensationExpenseRelatedToCommonStockOptionsRestrictedCommonStockAndCommonStockWarrantsDetail">
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_561" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable" xlink:label="loc_us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_565" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_561" xlink:to="loc_us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_565" xbrldt:contextElement="segment" xbrldt:closed="true" use="optional" order="1" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_IncomeStatementLocationAxis" xlink:label="loc_us-gaap_IncomeStatementLocationAxis_562" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_565" xlink:to="loc_us-gaap_IncomeStatementLocationAxis_562" use="optional" order="2" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_IncomeStatementLocationDomain" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_563" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_562" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_563" use="optional" order="3" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_IncomeStatementLocationDomain" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_563_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_562" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_563_default" use="optional" order="4" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_GeneralAndAdministrativeExpenseMember" xlink:label="loc_us-gaap_GeneralAndAdministrativeExpenseMember_568" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_563" xlink:to="loc_us-gaap_GeneralAndAdministrativeExpenseMember_568" use="optional" order="5" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ResearchAndDevelopmentExpenseMember" xlink:label="loc_us-gaap_ResearchAndDevelopmentExpenseMember_567" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_563" xlink:to="loc_us-gaap_ResearchAndDevelopmentExpenseMember_567" use="optional" order="6" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_LegalEntityAxis" xlink:label="loc_dei_LegalEntityAxis_564" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_565" xlink:to="loc_dei_LegalEntityAxis_564" use="optional" order="7" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_569" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_564" xlink:to="loc_dei_EntityDomain_569" use="optional" order="8" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_569_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_564" xlink:to="loc_dei_EntityDomain_569_default" use="optional" order="9" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensation" xlink:label="loc_us-gaap_ShareBasedCompensation_566" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_561" xlink:to="loc_us-gaap_ShareBasedCompensation_566" use="optional" order="10" />
</definitionLink>
<definitionLink xlink:type="extended" xlink:role="http://www.galectintherapeutics.com/role/WeightedAverageAssumptionsUsedToDetermineFairValueOfOptionsGrantedDetail">
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_574" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_573" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_574" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_573" xbrldt:contextElement="segment" xbrldt:closed="true" use="optional" order="1" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_LegalEntityAxis" xlink:label="loc_dei_LegalEntityAxis_572" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_573" xlink:to="loc_dei_LegalEntityAxis_572" use="optional" order="2" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_575" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_572" xlink:to="loc_dei_EntityDomain_575" use="optional" order="3" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_575_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_572" xlink:to="loc_dei_EntityDomain_575_default" use="optional" order="4" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate_579" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_574" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate_579" use="optional" order="5" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1_577" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_574" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1_577" use="optional" order="6" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate_578" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_574" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate_578" use="optional" order="7" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate_576" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_574" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate_576" use="optional" order="8" />
</definitionLink>
<definitionLink xlink:type="extended" xlink:role="http://www.galectintherapeutics.com/role/SummaryOfStockOptionActivityDetail">
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_584" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_583" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_584" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_583" xbrldt:contextElement="segment" xbrldt:closed="true" use="optional" order="1" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_LegalEntityAxis" xlink:label="loc_dei_LegalEntityAxis_582" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_583" xlink:to="loc_dei_LegalEntityAxis_582" use="optional" order="2" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_589" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_582" xlink:to="loc_dei_EntityDomain_589" use="optional" order="3" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_589_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_582" xlink:to="loc_dei_EntityDomain_589_default" use="optional" order="4" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAggregateIntrinsicValueAbstract" xlink:label="loc_galt_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAggregateIntrinsicValueAbstract_585" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_584" xlink:to="loc_galt_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAggregateIntrinsicValueAbstract_585" use="optional" order="5" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableIntrinsicValue1" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableIntrinsicValue1_592" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_galt_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAggregateIntrinsicValueAbstract_585" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableIntrinsicValue1_592" use="optional" order="6" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValue" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValue_591" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_galt_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAggregateIntrinsicValueAbstract_585" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValue_591" use="optional" order="7" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber_590" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_584" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber_590" use="optional" order="8" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_586" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_584" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_586" use="optional" order="9" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresInPeriod" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresInPeriod_595" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_586" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresInPeriod_595" use="optional" order="10" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross_594" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_586" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross_594" use="optional" order="11" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber_593" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_586" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber_593" use="optional" order="12" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_596" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_586" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_596" use="optional" order="13" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_587" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_584" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_587" use="optional" order="14" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice_603" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_587" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice_603" use="optional" order="15" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice_598" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_587" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice_598" use="optional" order="16" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExercisesInPeriodWeightedAverageExercisePrice" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExercisesInPeriodWeightedAverageExercisePrice_601" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_587" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExercisesInPeriodWeightedAverageExercisePrice_601" use="optional" order="17" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsForfeituresInPeriodWeightedAverageExercisePrice" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsForfeituresInPeriodWeightedAverageExercisePrice_600" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_587" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsForfeituresInPeriodWeightedAverageExercisePrice_600" use="optional" order="18" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice_599" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_587" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice_599" use="optional" order="19" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsWeightedAverageRemainingContractualTermAbstract" xlink:label="loc_galt_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsWeightedAverageRemainingContractualTermAbstract_588" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_584" xlink:to="loc_galt_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsWeightedAverageRemainingContractualTermAbstract_588" use="optional" order="20" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableWeightedAverageRemainingContractualTerm1" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableWeightedAverageRemainingContractualTerm1_605" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_galt_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsWeightedAverageRemainingContractualTermAbstract_588" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableWeightedAverageRemainingContractualTerm1_605" use="optional" order="21" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SharebasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeOutstandingOptionsWeightedAverageRemainingContractualTerm2" xlink:label="loc_us-gaap_SharebasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeOutstandingOptionsWeightedAverageRemainingContractualTerm2_604" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_galt_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsWeightedAverageRemainingContractualTermAbstract_588" xlink:to="loc_us-gaap_SharebasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeOutstandingOptionsWeightedAverageRemainingContractualTerm2_604" use="optional" order="22" />
</definitionLink>
<definitionLink xlink:type="extended" xlink:role="http://www.galectintherapeutics.com/role/SummaryOfAdditionalInformationRegardingOutstandingAndExercisableOptionsUnderStockBasedCompensationPlansDetail">
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_610" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_609" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_610" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_609" xbrldt:contextElement="segment" xbrldt:closed="true" use="optional" order="1" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_LegalEntityAxis" xlink:label="loc_dei_LegalEntityAxis_608" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_609" xlink:to="loc_dei_LegalEntityAxis_608" use="optional" order="2" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_613" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_608" xlink:to="loc_dei_EntityDomain_613" use="optional" order="3" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_613_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_608" xlink:to="loc_dei_EntityDomain_613_default" use="optional" order="4" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis" xlink:label="loc_us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis_611" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_609" xlink:to="loc_us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis_611" use="optional" order="5" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeDomain" xlink:label="loc_us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeDomain_612" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis_611" xlink:to="loc_us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeDomain_612" use="optional" order="6" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeDomain" xlink:label="loc_us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeDomain_612_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis_611" xlink:to="loc_us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeDomain_612_default" use="optional" order="7" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_ExercisePriceRangeFiveMember" xlink:label="loc_galt_ExercisePriceRangeFiveMember_625" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeDomain_612" xlink:to="loc_galt_ExercisePriceRangeFiveMember_625" use="optional" order="8" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_ExercisePriceRangeFourMember" xlink:label="loc_galt_ExercisePriceRangeFourMember_624" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeDomain_612" xlink:to="loc_galt_ExercisePriceRangeFourMember_624" use="optional" order="9" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_ExercisePriceRangeOneMember" xlink:label="loc_galt_ExercisePriceRangeOneMember_621" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeDomain_612" xlink:to="loc_galt_ExercisePriceRangeOneMember_621" use="optional" order="10" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_ExercisePriceRangeThreeMember" xlink:label="loc_galt_ExercisePriceRangeThreeMember_623" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeDomain_612" xlink:to="loc_galt_ExercisePriceRangeThreeMember_623" use="optional" order="11" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_ExercisePriceRangeTwoMember" xlink:label="loc_galt_ExercisePriceRangeTwoMember_622" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeDomain_612" xlink:to="loc_galt_ExercisePriceRangeTwoMember_622" use="optional" order="12" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SharebasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeExercisableOptionsWeightedAverageExercisePrice1" xlink:label="loc_us-gaap_SharebasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeExercisableOptionsWeightedAverageExercisePrice1_620" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_610" xlink:to="loc_us-gaap_SharebasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeExercisableOptionsWeightedAverageExercisePrice1_620" use="optional" order="13" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeLowerRangeLimit" xlink:label="loc_us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeLowerRangeLimit_614" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_610" xlink:to="loc_us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeLowerRangeLimit_614" use="optional" order="14" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeNumberOfExercisableOptions" xlink:label="loc_us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeNumberOfExercisableOptions_619" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_610" xlink:to="loc_us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeNumberOfExercisableOptions_619" use="optional" order="15" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeNumberOfOutstandingOptions" xlink:label="loc_us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeNumberOfOutstandingOptions_616" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_610" xlink:to="loc_us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeNumberOfOutstandingOptions_616" use="optional" order="16" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SharebasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeOutstandingOptionsWeightedAverageExercisePriceBeginningBalance1" xlink:label="loc_us-gaap_SharebasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeOutstandingOptionsWeightedAverageExercisePriceBeginningBalance1_618" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_610" xlink:to="loc_us-gaap_SharebasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeOutstandingOptionsWeightedAverageExercisePriceBeginningBalance1_618" use="optional" order="17" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SharebasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeOutstandingOptionsWeightedAverageRemainingContractualTerm2" xlink:label="loc_us-gaap_SharebasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeOutstandingOptionsWeightedAverageRemainingContractualTerm2_617" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_610" xlink:to="loc_us-gaap_SharebasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeOutstandingOptionsWeightedAverageRemainingContractualTerm2_617" use="optional" order="18" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeUpperRangeLimit" xlink:label="loc_us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeUpperRangeLimit_615" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_610" xlink:to="loc_us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeUpperRangeLimit_615" use="optional" order="19" />
</definitionLink>
<definitionLink xlink:type="extended" xlink:role="http://www.galectintherapeutics.com/role/LineOfCreditAdditionalInformationDetail">
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_LineOfCreditFacilityLineItems" xlink:label="loc_us-gaap_LineOfCreditFacilityLineItems_633" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1" xlink:label="loc_us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1_645" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_633" xlink:to="loc_us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1_645" use="optional" order="1" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ClassOfWarrantOrRightNumberOfSecuritiesCalledByWarrantsOrRights" xlink:label="loc_us-gaap_ClassOfWarrantOrRightNumberOfSecuritiesCalledByWarrantsOrRights_644" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_633" xlink:to="loc_us-gaap_ClassOfWarrantOrRightNumberOfSecuritiesCalledByWarrantsOrRights_644" use="optional" order="2" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_DebtInstrumentInterestRateStatedPercentage" xlink:label="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_643" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_633" xlink:to="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_643" use="optional" order="3" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_InterestExpenseDebt" xlink:label="loc_us-gaap_InterestExpenseDebt_652" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_633" xlink:to="loc_us-gaap_InterestExpenseDebt_652" use="optional" order="4" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_IssuanceOfWarrantsValue" xlink:label="loc_galt_IssuanceOfWarrantsValue_647" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_633" xlink:to="loc_galt_IssuanceOfWarrantsValue_647" use="optional" order="5" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity" xlink:label="loc_us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity_641" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_633" xlink:to="loc_us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity_641" use="optional" order="6" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_LineOfCreditFacilityTable" xlink:label="loc_us-gaap_LineOfCreditFacilityTable_634" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_633" xlink:to="loc_us-gaap_LineOfCreditFacilityTable_634" xbrldt:contextElement="segment" xbrldt:closed="true" use="optional" order="7" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_CreditFacilityAxis" xlink:label="loc_us-gaap_CreditFacilityAxis_629" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LineOfCreditFacilityTable_634" xlink:to="loc_us-gaap_CreditFacilityAxis_629" use="optional" order="8" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_CreditFacilityDomain" xlink:label="loc_us-gaap_CreditFacilityDomain_630" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_CreditFacilityAxis_629" xlink:to="loc_us-gaap_CreditFacilityDomain_630" use="optional" order="9" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_CreditFacilityDomain" xlink:label="loc_us-gaap_CreditFacilityDomain_630_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_CreditFacilityAxis_629" xlink:to="loc_us-gaap_CreditFacilityDomain_630_default" use="optional" order="10" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_UnsecuredLineOfCreditMember" xlink:label="loc_galt_UnsecuredLineOfCreditMember_638" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CreditFacilityDomain_630" xlink:to="loc_galt_UnsecuredLineOfCreditMember_638" use="optional" order="11" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_LegalEntityAxis" xlink:label="loc_dei_LegalEntityAxis_632" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LineOfCreditFacilityTable_634" xlink:to="loc_dei_LegalEntityAxis_632" use="optional" order="12" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_640" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_632" xlink:to="loc_dei_EntityDomain_640" use="optional" order="13" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_640_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_632" xlink:to="loc_dei_EntityDomain_640_default" use="optional" order="14" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementEquityComponentsAxis" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_635" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LineOfCreditFacilityTable_634" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_635" use="optional" order="15" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_EquityComponentDomain" xlink:label="loc_us-gaap_EquityComponentDomain_631" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_635" xlink:to="loc_us-gaap_EquityComponentDomain_631" use="optional" order="16" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_EquityComponentDomain" xlink:label="loc_us-gaap_EquityComponentDomain_631_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_635" xlink:to="loc_us-gaap_EquityComponentDomain_631_default" use="optional" order="17" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_WarrantMember" xlink:label="loc_us-gaap_WarrantMember_639" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_631" xlink:to="loc_us-gaap_WarrantMember_639" use="optional" order="18" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_TitleOfIndividualAxis" xlink:label="loc_us-gaap_TitleOfIndividualAxis_636" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LineOfCreditFacilityTable_634" xlink:to="loc_us-gaap_TitleOfIndividualAxis_636" use="optional" order="19" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_TitleOfIndividualWithRelationshipToEntityDomain" xlink:label="loc_us-gaap_TitleOfIndividualWithRelationshipToEntityDomain_637" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_TitleOfIndividualAxis_636" xlink:to="loc_us-gaap_TitleOfIndividualWithRelationshipToEntityDomain_637" use="optional" order="20" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_TitleOfIndividualWithRelationshipToEntityDomain" xlink:label="loc_us-gaap_TitleOfIndividualWithRelationshipToEntityDomain_637_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_TitleOfIndividualAxis_636" xlink:to="loc_us-gaap_TitleOfIndividualWithRelationshipToEntityDomain_637_default" use="optional" order="21" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_DirectorMember" xlink:label="loc_us-gaap_DirectorMember_653" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TitleOfIndividualWithRelationshipToEntityDomain_637" xlink:to="loc_us-gaap_DirectorMember_653" use="optional" order="22" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_OwnershipAxis" xlink:label="loc_srt_OwnershipAxis_322040" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LineOfCreditFacilityTable_634" xlink:to="loc_srt_OwnershipAxis_322040" use="optional" order="23" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_OwnershipDomain" xlink:label="loc_srt_OwnershipDomain_322041" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_OwnershipAxis_322040" xlink:to="loc_srt_OwnershipDomain_322041" use="optional" order="24" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_OwnershipDomain" xlink:label="loc_srt_OwnershipDomain_322041_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_OwnershipAxis_322040" xlink:to="loc_srt_OwnershipDomain_322041_default" use="optional" order="25" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_DirectorAndShareholderMember" xlink:label="loc_galt_DirectorAndShareholderMember_322056" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_OwnershipDomain_322041" xlink:to="loc_galt_DirectorAndShareholderMember_322056" use="optional" order="26" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_LinesOfCreditCurrent" xlink:label="loc_us-gaap_LinesOfCreditCurrent_646" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_633" xlink:to="loc_us-gaap_LinesOfCreditCurrent_646" use="optional" order="27" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_MinorityInterestOwnershipPercentageByNoncontrollingOwners" xlink:label="loc_us-gaap_MinorityInterestOwnershipPercentageByNoncontrollingOwners_642" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_633" xlink:to="loc_us-gaap_MinorityInterestOwnershipPercentageByNoncontrollingOwners_642" use="optional" order="28" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate_650" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_633" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate_650" use="optional" order="29" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1_648" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_633" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1_648" use="optional" order="30" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate_649" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_633" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate_649" use="optional" order="31" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate_651" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_633" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate_651" use="optional" order="32" />
</definitionLink>
<definitionLink xlink:type="extended" xlink:role="http://www.galectintherapeutics.com/role/ComputationOfDilutedNetLossPerShareDetail">
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_ScheduleOfComputationOfBasicAndDilutedEarningsPerCommonShareLineItems" xlink:label="loc_galt_ScheduleOfComputationOfBasicAndDilutedEarningsPerCommonShareLineItems_657" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_EarningsPerShareBasicAndDiluted" xlink:label="loc_us-gaap_EarningsPerShareBasicAndDiluted_664" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_galt_ScheduleOfComputationOfBasicAndDilutedEarningsPerCommonShareLineItems_657" xlink:to="loc_us-gaap_EarningsPerShareBasicAndDiluted_664" use="optional" order="1" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_NetIncomeLoss" xlink:label="loc_us-gaap_NetIncomeLoss_660" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_galt_ScheduleOfComputationOfBasicAndDilutedEarningsPerCommonShareLineItems_657" xlink:to="loc_us-gaap_NetIncomeLoss_660" use="optional" order="2" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic" xlink:label="loc_us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic_663" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_galt_ScheduleOfComputationOfBasicAndDilutedEarningsPerCommonShareLineItems_657" xlink:to="loc_us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic_663" use="optional" order="3" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_PreferredStockDividendsIncomeStatementImpact" xlink:label="loc_us-gaap_PreferredStockDividendsIncomeStatementImpact_661" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_galt_ScheduleOfComputationOfBasicAndDilutedEarningsPerCommonShareLineItems_657" xlink:to="loc_us-gaap_PreferredStockDividendsIncomeStatementImpact_661" use="optional" order="4" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_ScheduleOfComputationOfBasicAndDilutedEarningsPerCommonShareTable" xlink:label="loc_galt_ScheduleOfComputationOfBasicAndDilutedEarningsPerCommonShareTable_658" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_galt_ScheduleOfComputationOfBasicAndDilutedEarningsPerCommonShareLineItems_657" xlink:to="loc_galt_ScheduleOfComputationOfBasicAndDilutedEarningsPerCommonShareTable_658" xbrldt:contextElement="segment" xbrldt:closed="true" use="optional" order="5" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_LegalEntityAxis" xlink:label="loc_dei_LegalEntityAxis_656" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_galt_ScheduleOfComputationOfBasicAndDilutedEarningsPerCommonShareTable_658" xlink:to="loc_dei_LegalEntityAxis_656" use="optional" order="6" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_659" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_656" xlink:to="loc_dei_EntityDomain_659" use="optional" order="7" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_659_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_656" xlink:to="loc_dei_EntityDomain_659_default" use="optional" order="8" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_WeightedAverageNumberOfShareOutstandingBasicAndDiluted" xlink:label="loc_us-gaap_WeightedAverageNumberOfShareOutstandingBasicAndDiluted_665" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_galt_ScheduleOfComputationOfBasicAndDilutedEarningsPerCommonShareLineItems_657" xlink:to="loc_us-gaap_WeightedAverageNumberOfShareOutstandingBasicAndDiluted_665" use="optional" order="9" />
</definitionLink>
<definitionLink xlink:type="extended" xlink:role="http://www.galectintherapeutics.com/role/DilutiveSharesWhichCouldExistPursuantToExerciseOfOutstandingStockInstrumentsAndWhichWereNotIncludedInCalculationDetail">
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_669" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount_673" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_669" xlink:to="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount_673" use="optional" order="1" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable" xlink:label="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_672" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_669" xlink:to="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_672" xbrldt:contextElement="segment" xbrldt:closed="true" use="optional" order="2" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_668" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_672" xlink:to="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_668" use="optional" order="3" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_AntidilutiveSecuritiesNameDomain" xlink:label="loc_us-gaap_AntidilutiveSecuritiesNameDomain_670" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_668" xlink:to="loc_us-gaap_AntidilutiveSecuritiesNameDomain_670" use="optional" order="4" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_AntidilutiveSecuritiesNameDomain" xlink:label="loc_us-gaap_AntidilutiveSecuritiesNameDomain_670_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_668" xlink:to="loc_us-gaap_AntidilutiveSecuritiesNameDomain_670_default" use="optional" order="5" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_EmployeeStockOptionMember" xlink:label="loc_us-gaap_EmployeeStockOptionMember_675" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_670" xlink:to="loc_us-gaap_EmployeeStockOptionMember_675" use="optional" order="6" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StockCompensationPlanMember" xlink:label="loc_us-gaap_StockCompensationPlanMember_676" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_670" xlink:to="loc_us-gaap_StockCompensationPlanMember_676" use="optional" order="7" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_WarrantMember" xlink:label="loc_us-gaap_WarrantMember_674" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_670" xlink:to="loc_us-gaap_WarrantMember_674" use="optional" order="8" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_LegalEntityAxis" xlink:label="loc_dei_LegalEntityAxis_671" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_672" xlink:to="loc_dei_LegalEntityAxis_671" use="optional" order="9" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_677" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_671" xlink:to="loc_dei_EntityDomain_677" use="optional" order="10" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_677_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_671" xlink:to="loc_dei_EntityDomain_677_default" use="optional" order="11" />
</definitionLink>
<definitionLink xlink:type="extended" xlink:role="http://www.galectintherapeutics.com/role/CommitmentsAndContingenciesAdditionalInformationDetail">
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_CommitmentAndContingenciesLineItems" xlink:label="loc_galt_CommitmentAndContingenciesLineItems_680" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_CommitmentAndContingenciesTable" xlink:label="loc_galt_CommitmentAndContingenciesTable_681" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_galt_CommitmentAndContingenciesLineItems_680" xlink:to="loc_galt_CommitmentAndContingenciesTable_681" xbrldt:contextElement="segment" xbrldt:closed="true" use="optional" order="1" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_LegalEntityAxis" xlink:label="loc_dei_LegalEntityAxis_682" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_galt_CommitmentAndContingenciesTable_681" xlink:to="loc_dei_LegalEntityAxis_682" use="optional" order="2" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_690" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_682" xlink:to="loc_dei_EntityDomain_690" use="optional" order="3" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_690_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_682" xlink:to="loc_dei_EntityDomain_690_default" use="optional" order="4" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_LocationAxis" xlink:label="loc_galt_LocationAxis_683" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_galt_CommitmentAndContingenciesTable_681" xlink:to="loc_galt_LocationAxis_683" use="optional" order="5" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_LocationsDomain" xlink:label="loc_galt_LocationsDomain_684" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_galt_LocationAxis_683" xlink:to="loc_galt_LocationsDomain_684" use="optional" order="6" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_LocationsDomain" xlink:label="loc_galt_LocationsDomain_684_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_galt_LocationAxis_683" xlink:to="loc_galt_LocationsDomain_684_default" use="optional" order="7" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_NorcrossMember" xlink:label="loc_galt_NorcrossMember_691" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_galt_LocationsDomain_684" xlink:to="loc_galt_NorcrossMember_691" use="optional" order="8" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_LeaseAndRentalExpense" xlink:label="loc_us-gaap_LeaseAndRentalExpense_689" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_galt_CommitmentAndContingenciesLineItems_680" xlink:to="loc_us-gaap_LeaseAndRentalExpense_689" use="optional" order="9" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_LeaseDeposits" xlink:label="loc_galt_LeaseDeposits_688" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_galt_CommitmentAndContingenciesLineItems_680" xlink:to="loc_galt_LeaseDeposits_688" use="optional" order="10" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_LeaseExpirationDate1" xlink:label="loc_us-gaap_LeaseExpirationDate1_686" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_galt_CommitmentAndContingenciesLineItems_680" xlink:to="loc_us-gaap_LeaseExpirationDate1_686" use="optional" order="11" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_OperatingLeasesMonthlyBaseRentExpense" xlink:label="loc_galt_OperatingLeasesMonthlyBaseRentExpense_687" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_galt_CommitmentAndContingenciesLineItems_680" xlink:to="loc_galt_OperatingLeasesMonthlyBaseRentExpense_687" use="optional" order="12" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_LesseeOperatingLeaseTermOfContract" xlink:label="loc_us-gaap_LesseeOperatingLeaseTermOfContract_322014" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_galt_CommitmentAndContingenciesLineItems_680" xlink:to="loc_us-gaap_LesseeOperatingLeaseTermOfContract_322014" use="optional" order="13" />
</definitionLink>
<definitionLink xlink:type="extended" xlink:role="http://www.galectintherapeutics.com/role/FutureMinimumPaymentsOfOperatingLeaseDetail">
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_OperatingLeasedAssetsLineItems" xlink:label="loc_us-gaap_OperatingLeasedAssetsLineItems_695" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_OperatingLeasesFutureMinimumPaymentsDue" xlink:label="loc_us-gaap_OperatingLeasesFutureMinimumPaymentsDue_699" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLeasedAssetsLineItems_695" xlink:to="loc_us-gaap_OperatingLeasesFutureMinimumPaymentsDue_699" use="optional" order="1" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ScheduleOfOperatingLeasedAssetsTable" xlink:label="loc_us-gaap_ScheduleOfOperatingLeasedAssetsTable_696" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_OperatingLeasedAssetsLineItems_695" xlink:to="loc_us-gaap_ScheduleOfOperatingLeasedAssetsTable_696" xbrldt:contextElement="segment" xbrldt:closed="true" use="optional" order="2" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_LegalEntityAxis" xlink:label="loc_dei_LegalEntityAxis_694" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfOperatingLeasedAssetsTable_696" xlink:to="loc_dei_LegalEntityAxis_694" use="optional" order="3" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_697" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_694" xlink:to="loc_dei_EntityDomain_697" use="optional" order="4" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_697_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_694" xlink:to="loc_dei_EntityDomain_697_default" use="optional" order="5" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_OperatingLeasesFutureMinimumPaymentsDueCurrent" xlink:label="loc_us-gaap_OperatingLeasesFutureMinimumPaymentsDueCurrent_320921" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLeasedAssetsLineItems_695" xlink:to="loc_us-gaap_OperatingLeasesFutureMinimumPaymentsDueCurrent_320921" use="optional" order="6" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_OperatingLeasesFutureMinimumPaymentsDueInTwoYears" xlink:label="loc_us-gaap_OperatingLeasesFutureMinimumPaymentsDueInTwoYears_320923" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLeasedAssetsLineItems_695" xlink:to="loc_us-gaap_OperatingLeasesFutureMinimumPaymentsDueInTwoYears_320923" use="optional" order="7" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_OperatingLeasesFutureMinimumPaymentsDueInThreeYears" xlink:label="loc_us-gaap_OperatingLeasesFutureMinimumPaymentsDueInThreeYears_320924" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLeasedAssetsLineItems_695" xlink:to="loc_us-gaap_OperatingLeasesFutureMinimumPaymentsDueInThreeYears_320924" use="optional" order="8" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_OperatingLeasesFutureMinimumPaymentsDueInFourYears" xlink:label="loc_us-gaap_OperatingLeasesFutureMinimumPaymentsDueInFourYears_320925" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLeasedAssetsLineItems_695" xlink:to="loc_us-gaap_OperatingLeasesFutureMinimumPaymentsDueInFourYears_320925" use="optional" order="9" />
</definitionLink>
<definitionLink xlink:type="extended" xlink:role="http://www.galectintherapeutics.com/role/GalectinScienceLlcAdditionalInformationDetail">
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_InvestmentsInAndAdvancesToAffiliatesLineItems" xlink:label="loc_us-gaap_InvestmentsInAndAdvancesToAffiliatesLineItems_706" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_FairValueOfAssetContributedToJointVenture" xlink:label="loc_galt_FairValueOfAssetContributedToJointVenture_713" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InvestmentsInAndAdvancesToAffiliatesLineItems_706" xlink:to="loc_galt_FairValueOfAssetContributedToJointVenture_713" use="optional" order="1" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_IncomeLossFromEquityMethodInvestments" xlink:label="loc_us-gaap_IncomeLossFromEquityMethodInvestments_716" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InvestmentsInAndAdvancesToAffiliatesLineItems_706" xlink:to="loc_us-gaap_IncomeLossFromEquityMethodInvestments_716" use="optional" order="2" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_InvestmentsInAndAdvancesToAffiliatesTable" xlink:label="loc_us-gaap_InvestmentsInAndAdvancesToAffiliatesTable_707" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_InvestmentsInAndAdvancesToAffiliatesLineItems_706" xlink:to="loc_us-gaap_InvestmentsInAndAdvancesToAffiliatesTable_707" xbrldt:contextElement="segment" xbrldt:closed="true" use="optional" order="3" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_CounterpartyNameAxis" xlink:label="loc_srt_CounterpartyNameAxis_702" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_InvestmentsInAndAdvancesToAffiliatesTable_707" xlink:to="loc_srt_CounterpartyNameAxis_702" use="optional" order="4" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_RepurchaseAgreementCounterpartyNameDomain" xlink:label="loc_srt_RepurchaseAgreementCounterpartyNameDomain_709" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_CounterpartyNameAxis_702" xlink:to="loc_srt_RepurchaseAgreementCounterpartyNameDomain_709" use="optional" order="5" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_RepurchaseAgreementCounterpartyNameDomain" xlink:label="loc_srt_RepurchaseAgreementCounterpartyNameDomain_709_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_CounterpartyNameAxis_702" xlink:to="loc_srt_RepurchaseAgreementCounterpartyNameDomain_709_default" use="optional" order="6" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_SbhSciencesIncMember" xlink:label="loc_galt_SbhSciencesIncMember_719" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RepurchaseAgreementCounterpartyNameDomain_709" xlink:to="loc_galt_SbhSciencesIncMember_719" use="optional" order="7" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_704" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_InvestmentsInAndAdvancesToAffiliatesTable_707" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_704" use="optional" order="8" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_705" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_704" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_705" use="optional" order="9" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_705_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_704" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_705_default" use="optional" order="10" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_InProcessResearchAndDevelopmentMember" xlink:label="loc_us-gaap_InProcessResearchAndDevelopmentMember_711" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_705" xlink:to="loc_us-gaap_InProcessResearchAndDevelopmentMember_711" use="optional" order="11" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_OwnershipAxis" xlink:label="loc_srt_OwnershipAxis_708" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_InvestmentsInAndAdvancesToAffiliatesTable_707" xlink:to="loc_srt_OwnershipAxis_708" use="optional" order="12" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_OwnershipDomain" xlink:label="loc_srt_OwnershipDomain_703" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_OwnershipAxis_708" xlink:to="loc_srt_OwnershipDomain_703" use="optional" order="13" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_OwnershipDomain" xlink:label="loc_srt_OwnershipDomain_703_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_OwnershipAxis_708" xlink:to="loc_srt_OwnershipDomain_703_default" use="optional" order="14" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_GalectinSciencesLlcMember" xlink:label="loc_galt_GalectinSciencesLlcMember_710" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_OwnershipDomain_703" xlink:to="loc_galt_GalectinSciencesLlcMember_710" use="optional" order="15" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_MinorityInterestOwnershipPercentageByParent" xlink:label="loc_us-gaap_MinorityInterestOwnershipPercentageByParent_715" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InvestmentsInAndAdvancesToAffiliatesLineItems_706" xlink:to="loc_us-gaap_MinorityInterestOwnershipPercentageByParent_715" use="optional" order="16" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_PaymentsToAcquireBusinessesAndInterestInAffiliates" xlink:label="loc_us-gaap_PaymentsToAcquireBusinessesAndInterestInAffiliates_714" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InvestmentsInAndAdvancesToAffiliatesLineItems_706" xlink:to="loc_us-gaap_PaymentsToAcquireBusinessesAndInterestInAffiliates_714" use="optional" order="17" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_PaymentsToAcquireEquityMethodInvestments" xlink:label="loc_us-gaap_PaymentsToAcquireEquityMethodInvestments_712" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InvestmentsInAndAdvancesToAffiliatesLineItems_706" xlink:to="loc_us-gaap_PaymentsToAcquireEquityMethodInvestments_712" use="optional" order="18" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ResearchAndDevelopmentInProcess" xlink:label="loc_us-gaap_ResearchAndDevelopmentInProcess_717" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InvestmentsInAndAdvancesToAffiliatesLineItems_706" xlink:to="loc_us-gaap_ResearchAndDevelopmentInProcess_717" use="optional" order="19" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SubsidiaryOrEquityMethodInvesteeCumulativePercentageOwnershipAfterAllTransactions" xlink:label="loc_us-gaap_SubsidiaryOrEquityMethodInvesteeCumulativePercentageOwnershipAfterAllTransactions_718" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InvestmentsInAndAdvancesToAffiliatesLineItems_706" xlink:to="loc_us-gaap_SubsidiaryOrEquityMethodInvesteeCumulativePercentageOwnershipAfterAllTransactions_718" use="optional" order="20" />
</definitionLink>
<definitionLink xlink:type="extended" xlink:role="http://www.galectintherapeutics.com/role/IncomeTaxesAdditionalInformationDetail">
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_IncomeTaxesLineItems" xlink:label="loc_galt_IncomeTaxesLineItems_726" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_DeferredTaxAssetsValuationAllowancePercentage" xlink:label="loc_galt_DeferredTaxAssetsValuationAllowancePercentage_739" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_galt_IncomeTaxesLineItems_726" xlink:to="loc_galt_DeferredTaxAssetsValuationAllowancePercentage_739" use="optional" order="1" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_DeferredTaxExpenseFromStockOptionsExercised" xlink:label="loc_us-gaap_DeferredTaxExpenseFromStockOptionsExercised_736" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_galt_IncomeTaxesLineItems_726" xlink:to="loc_us-gaap_DeferredTaxExpenseFromStockOptionsExercised_736" use="optional" order="2" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate" xlink:label="loc_us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate_733" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_galt_IncomeTaxesLineItems_726" xlink:to="loc_us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate_733" use="optional" order="3" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_IncomeTaxesTable" xlink:label="loc_galt_IncomeTaxesTable_727" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_galt_IncomeTaxesLineItems_726" xlink:to="loc_galt_IncomeTaxesTable_727" xbrldt:contextElement="segment" xbrldt:closed="true" use="optional" order="4" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_IncomeTaxAuthorityAxis" xlink:label="loc_us-gaap_IncomeTaxAuthorityAxis_722" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_galt_IncomeTaxesTable_727" xlink:to="loc_us-gaap_IncomeTaxAuthorityAxis_722" use="optional" order="5" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_IncomeTaxAuthorityDomain" xlink:label="loc_us-gaap_IncomeTaxAuthorityDomain_723" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeTaxAuthorityAxis_722" xlink:to="loc_us-gaap_IncomeTaxAuthorityDomain_723" use="optional" order="6" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_IncomeTaxAuthorityDomain" xlink:label="loc_us-gaap_IncomeTaxAuthorityDomain_723_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeTaxAuthorityAxis_722" xlink:to="loc_us-gaap_IncomeTaxAuthorityDomain_723_default" use="optional" order="7" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StateAndLocalJurisdictionMember" xlink:label="loc_us-gaap_StateAndLocalJurisdictionMember_731" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxAuthorityDomain_723" xlink:to="loc_us-gaap_StateAndLocalJurisdictionMember_731" use="optional" order="8" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_IncomeTaxAuthorityNameAxis" xlink:label="loc_us-gaap_IncomeTaxAuthorityNameAxis_724" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_galt_IncomeTaxesTable_727" xlink:to="loc_us-gaap_IncomeTaxAuthorityNameAxis_724" use="optional" order="9" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_IncomeTaxAuthorityNameDomain" xlink:label="loc_us-gaap_IncomeTaxAuthorityNameDomain_725" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeTaxAuthorityNameAxis_724" xlink:to="loc_us-gaap_IncomeTaxAuthorityNameDomain_725" use="optional" order="10" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_IncomeTaxAuthorityNameDomain" xlink:label="loc_us-gaap_IncomeTaxAuthorityNameDomain_725_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeTaxAuthorityNameAxis_724" xlink:to="loc_us-gaap_IncomeTaxAuthorityNameDomain_725_default" use="optional" order="11" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_InternalRevenueServiceIRSMember" xlink:label="loc_us-gaap_InternalRevenueServiceIRSMember_732" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxAuthorityNameDomain_725" xlink:to="loc_us-gaap_InternalRevenueServiceIRSMember_732" use="optional" order="12" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_LegalEntityAxis" xlink:label="loc_dei_LegalEntityAxis_728" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_galt_IncomeTaxesTable_727" xlink:to="loc_dei_LegalEntityAxis_728" use="optional" order="13" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_740" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_728" xlink:to="loc_dei_EntityDomain_740" use="optional" order="14" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_740_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_728" xlink:to="loc_dei_EntityDomain_740_default" use="optional" order="15" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementScenarioAxis" xlink:label="loc_us-gaap_StatementScenarioAxis_730" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_galt_IncomeTaxesTable_727" xlink:to="loc_us-gaap_StatementScenarioAxis_730" use="optional" order="16" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="loc_us-gaap_ScenarioUnspecifiedDomain_729" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementScenarioAxis_730" xlink:to="loc_us-gaap_ScenarioUnspecifiedDomain_729" use="optional" order="17" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="loc_us-gaap_ScenarioUnspecifiedDomain_729_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementScenarioAxis_730" xlink:to="loc_us-gaap_ScenarioUnspecifiedDomain_729_default" use="optional" order="18" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ScenarioPlanMember" xlink:label="loc_us-gaap_ScenarioPlanMember_741" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScenarioUnspecifiedDomain_729" xlink:to="loc_us-gaap_ScenarioPlanMember_741" use="optional" order="19" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_IncomeTaxReconciliationTaxCreditsResearch" xlink:label="loc_us-gaap_IncomeTaxReconciliationTaxCreditsResearch_737" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_galt_IncomeTaxesLineItems_726" xlink:to="loc_us-gaap_IncomeTaxReconciliationTaxCreditsResearch_737" use="optional" order="20" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_OperatingLossCarryforwards" xlink:label="loc_us-gaap_OperatingLossCarryforwards_734" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_galt_IncomeTaxesLineItems_726" xlink:to="loc_us-gaap_OperatingLossCarryforwards_734" use="optional" order="21" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_OperatingLossCarryforwardsExpirationYear" xlink:label="loc_galt_OperatingLossCarryforwardsExpirationYear_735" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_galt_IncomeTaxesLineItems_726" xlink:to="loc_galt_OperatingLossCarryforwardsExpirationYear_735" use="optional" order="22" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_ResearchAndDevelopmentCreditsExpirationYear" xlink:label="loc_galt_ResearchAndDevelopmentCreditsExpirationYear_738" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_galt_IncomeTaxesLineItems_726" xlink:to="loc_galt_ResearchAndDevelopmentCreditsExpirationYear_738" use="optional" order="23" />
</definitionLink>
<definitionLink xlink:type="extended" xlink:role="http://www.galectintherapeutics.com/role/ComponentsOfNetDeferredTaxAssetsDetail">
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_ComponentsOfDeferredTaxAssetsAndLiabilitiesLineItems" xlink:label="loc_galt_ComponentsOfDeferredTaxAssetsAndLiabilitiesLineItems_744" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_ComponentsOfDeferredTaxAssetsAndLiabilitiesTable" xlink:label="loc_galt_ComponentsOfDeferredTaxAssetsAndLiabilitiesTable_745" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_galt_ComponentsOfDeferredTaxAssetsAndLiabilitiesLineItems_744" xlink:to="loc_galt_ComponentsOfDeferredTaxAssetsAndLiabilitiesTable_745" xbrldt:contextElement="segment" xbrldt:closed="true" use="optional" order="1" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_LegalEntityAxis" xlink:label="loc_dei_LegalEntityAxis_746" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_galt_ComponentsOfDeferredTaxAssetsAndLiabilitiesTable_745" xlink:to="loc_dei_LegalEntityAxis_746" use="optional" order="2" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_753" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_746" xlink:to="loc_dei_EntityDomain_753" use="optional" order="3" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_753_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_746" xlink:to="loc_dei_EntityDomain_753_default" use="optional" order="4" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_DeferredTaxAssetsGross" xlink:label="loc_us-gaap_DeferredTaxAssetsGross_750" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_galt_ComponentsOfDeferredTaxAssetsAndLiabilitiesLineItems_744" xlink:to="loc_us-gaap_DeferredTaxAssetsGross_750" use="optional" order="5" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_DeferredTaxAssetsNet" xlink:label="loc_us-gaap_DeferredTaxAssetsNet_752" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_galt_ComponentsOfDeferredTaxAssetsAndLiabilitiesLineItems_744" xlink:to="loc_us-gaap_DeferredTaxAssetsNet_752" use="optional" order="6" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_DeferredTaxAssetsOperatingLossCarryforwards" xlink:label="loc_us-gaap_DeferredTaxAssetsOperatingLossCarryforwards_747" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_galt_ComponentsOfDeferredTaxAssetsAndLiabilitiesLineItems_744" xlink:to="loc_us-gaap_DeferredTaxAssetsOperatingLossCarryforwards_747" use="optional" order="7" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_DeferredTaxAssetsOther" xlink:label="loc_us-gaap_DeferredTaxAssetsOther_749" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_galt_ComponentsOfDeferredTaxAssetsAndLiabilitiesLineItems_744" xlink:to="loc_us-gaap_DeferredTaxAssetsOther_749" use="optional" order="8" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_DeferredTaxAssetsTaxCreditCarryforwards" xlink:label="loc_us-gaap_DeferredTaxAssetsTaxCreditCarryforwards_748" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_galt_ComponentsOfDeferredTaxAssetsAndLiabilitiesLineItems_744" xlink:to="loc_us-gaap_DeferredTaxAssetsTaxCreditCarryforwards_748" use="optional" order="9" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_DeferredTaxAssetsValuationAllowance" xlink:label="loc_us-gaap_DeferredTaxAssetsValuationAllowance_751" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_galt_ComponentsOfDeferredTaxAssetsAndLiabilitiesLineItems_744" xlink:to="loc_us-gaap_DeferredTaxAssetsValuationAllowance_751" use="optional" order="10" />
</definitionLink>
<definitionLink xlink:type="extended" xlink:role="http://www.galectintherapeutics.com/role/PrimaryFactorsAffectingIncomeTaxRatesDetail">
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_ReconciliationOfStatutoryTaxRateLineItems" xlink:label="loc_galt_ReconciliationOfStatutoryTaxRateLineItems_757" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_EffectiveIncomeTaxRateContinuingOperations" xlink:label="loc_us-gaap_EffectiveIncomeTaxRateContinuingOperations_767" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_galt_ReconciliationOfStatutoryTaxRateLineItems_757" xlink:to="loc_us-gaap_EffectiveIncomeTaxRateContinuingOperations_767" use="optional" order="1" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate" xlink:label="loc_us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate_760" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_galt_ReconciliationOfStatutoryTaxRateLineItems_757" xlink:to="loc_us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate_760" use="optional" order="2" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_EffectiveIncomeTaxRateReconciliationChangeInDeferredTaxAssetsValuationAllowance" xlink:label="loc_us-gaap_EffectiveIncomeTaxRateReconciliationChangeInDeferredTaxAssetsValuationAllowance_766" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_galt_ReconciliationOfStatutoryTaxRateLineItems_757" xlink:to="loc_us-gaap_EffectiveIncomeTaxRateReconciliationChangeInDeferredTaxAssetsValuationAllowance_766" use="optional" order="3" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_EffectiveIncomeTaxRateReconciliationChangeInEnactedTaxRate" xlink:label="loc_us-gaap_EffectiveIncomeTaxRateReconciliationChangeInEnactedTaxRate_763" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_galt_ReconciliationOfStatutoryTaxRateLineItems_757" xlink:to="loc_us-gaap_EffectiveIncomeTaxRateReconciliationChangeInEnactedTaxRate_763" use="optional" order="4" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_EffectiveIncomeTaxRateReconciliationExpiringStateNonOperatingLoss" xlink:label="loc_galt_EffectiveIncomeTaxRateReconciliationExpiringStateNonOperatingLoss_765" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_galt_ReconciliationOfStatutoryTaxRateLineItems_757" xlink:to="loc_galt_EffectiveIncomeTaxRateReconciliationExpiringStateNonOperatingLoss_765" use="optional" order="5" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_EffectiveIncomeTaxRateReconciliationOtherAdjustments" xlink:label="loc_us-gaap_EffectiveIncomeTaxRateReconciliationOtherAdjustments_764" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_galt_ReconciliationOfStatutoryTaxRateLineItems_757" xlink:to="loc_us-gaap_EffectiveIncomeTaxRateReconciliationOtherAdjustments_764" use="optional" order="6" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_EffectiveIncomeTaxRateReconciliationPermanentDifferences" xlink:label="loc_galt_EffectiveIncomeTaxRateReconciliationPermanentDifferences_762" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_galt_ReconciliationOfStatutoryTaxRateLineItems_757" xlink:to="loc_galt_EffectiveIncomeTaxRateReconciliationPermanentDifferences_762" use="optional" order="7" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_EffectiveIncomeTaxRateReconciliationStateAndLocalIncomeTaxes" xlink:label="loc_us-gaap_EffectiveIncomeTaxRateReconciliationStateAndLocalIncomeTaxes_761" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_galt_ReconciliationOfStatutoryTaxRateLineItems_757" xlink:to="loc_us-gaap_EffectiveIncomeTaxRateReconciliationStateAndLocalIncomeTaxes_761" use="optional" order="8" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_ReconciliationOfStatutoryTaxRateTable" xlink:label="loc_galt_ReconciliationOfStatutoryTaxRateTable_758" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_galt_ReconciliationOfStatutoryTaxRateLineItems_757" xlink:to="loc_galt_ReconciliationOfStatutoryTaxRateTable_758" xbrldt:contextElement="segment" xbrldt:closed="true" use="optional" order="9" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_LegalEntityAxis" xlink:label="loc_dei_LegalEntityAxis_756" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_galt_ReconciliationOfStatutoryTaxRateTable_758" xlink:to="loc_dei_LegalEntityAxis_756" use="optional" order="10" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_759" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_756" xlink:to="loc_dei_EntityDomain_759" use="optional" order="11" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_759_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_756" xlink:to="loc_dei_EntityDomain_759_default" use="optional" order="12" />
</definitionLink>
<definitionLink xlink:type="extended" xlink:role="http://www.galectintherapeutics.com/role/SubsequentEventsAdditionalInformationDetail">
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementLineItems" xlink:label="loc_us-gaap_StatementLineItems_320935" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementTable" xlink:label="loc_us-gaap_StatementTable_320932" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_320935" xlink:to="loc_us-gaap_StatementTable_320932" xbrldt:contextElement="segment" xbrldt:closed="true" use="optional" order="1" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementClassOfStockAxis" xlink:label="loc_us-gaap_StatementClassOfStockAxis_320933" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_320932" xlink:to="loc_us-gaap_StatementClassOfStockAxis_320933" use="optional" order="2" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ClassOfStockDomain" xlink:label="loc_us-gaap_ClassOfStockDomain_320934" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_320933" xlink:to="loc_us-gaap_ClassOfStockDomain_320934" use="optional" order="3" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ClassOfStockDomain" xlink:label="loc_us-gaap_ClassOfStockDomain_320934_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_320933" xlink:to="loc_us-gaap_ClassOfStockDomain_320934_default" use="optional" order="4" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_CommonStockMember" xlink:label="loc_us-gaap_CommonStockMember_320936" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_320934" xlink:to="loc_us-gaap_CommonStockMember_320936" use="optional" order="5" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_SeriesB1B2AndB3ConvertiblePreferredStockMember" xlink:label="loc_galt_SeriesB1B2AndB3ConvertiblePreferredStockMember_321687" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_320934" xlink:to="loc_galt_SeriesB1B2AndB3ConvertiblePreferredStockMember_321687" use="optional" order="6" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SubsequentEventTypeAxis" xlink:label="loc_us-gaap_SubsequentEventTypeAxis_321666" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_320932" xlink:to="loc_us-gaap_SubsequentEventTypeAxis_321666" use="optional" order="7" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SubsequentEventTypeDomain" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_321667" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_SubsequentEventTypeAxis_321666" xlink:to="loc_us-gaap_SubsequentEventTypeDomain_321667" use="optional" order="8" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SubsequentEventTypeDomain" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_321667_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_SubsequentEventTypeAxis_321666" xlink:to="loc_us-gaap_SubsequentEventTypeDomain_321667_default" use="optional" order="9" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SubsequentEventMember" xlink:label="loc_us-gaap_SubsequentEventMember_321668" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventTypeDomain_321667" xlink:to="loc_us-gaap_SubsequentEventMember_321668" use="optional" order="10" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_MeasurementInputTypeAxis" xlink:label="loc_us-gaap_MeasurementInputTypeAxis_321670" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_320932" xlink:to="loc_us-gaap_MeasurementInputTypeAxis_321670" use="optional" order="11" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_MeasurementInputTypeDomain" xlink:label="loc_us-gaap_MeasurementInputTypeDomain_321671" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_MeasurementInputTypeAxis_321670" xlink:to="loc_us-gaap_MeasurementInputTypeDomain_321671" use="optional" order="12" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_MeasurementInputTypeDomain" xlink:label="loc_us-gaap_MeasurementInputTypeDomain_321671_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_MeasurementInputTypeAxis_321670" xlink:to="loc_us-gaap_MeasurementInputTypeDomain_321671_default" use="optional" order="13" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_MeasurementInputExpectedTermMember" xlink:label="loc_us-gaap_MeasurementInputExpectedTermMember_321672" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MeasurementInputTypeDomain_321671" xlink:to="loc_us-gaap_MeasurementInputExpectedTermMember_321672" use="optional" order="14" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_MeasurementInputPriceVolatilityMember" xlink:label="loc_us-gaap_MeasurementInputPriceVolatilityMember_321673" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MeasurementInputTypeDomain_321671" xlink:to="loc_us-gaap_MeasurementInputPriceVolatilityMember_321673" use="optional" order="15" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_MeasurementInputRiskFreeInterestRateMember" xlink:label="loc_us-gaap_MeasurementInputRiskFreeInterestRateMember_321674" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MeasurementInputTypeDomain_321671" xlink:to="loc_us-gaap_MeasurementInputRiskFreeInterestRateMember_321674" use="optional" order="16" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_MeasurementInputExpectedDividendRateMember" xlink:label="loc_us-gaap_MeasurementInputExpectedDividendRateMember_321675" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MeasurementInputTypeDomain_321671" xlink:to="loc_us-gaap_MeasurementInputExpectedDividendRateMember_321675" use="optional" order="17" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_RangeAxis" xlink:label="loc_srt_RangeAxis_321676" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_320932" xlink:to="loc_srt_RangeAxis_321676" use="optional" order="18" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_RangeMember" xlink:label="loc_srt_RangeMember_321677" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_321676" xlink:to="loc_srt_RangeMember_321677" use="optional" order="19" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_RangeMember" xlink:label="loc_srt_RangeMember_321677_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_321676" xlink:to="loc_srt_RangeMember_321677_default" use="optional" order="20" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_MaximumMember" xlink:label="loc_srt_MaximumMember_321678" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_321677" xlink:to="loc_srt_MaximumMember_321678" use="optional" order="21" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_MinimumMember" xlink:label="loc_srt_MinimumMember_321679" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_321677" xlink:to="loc_srt_MinimumMember_321679" use="optional" order="22" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_ModificationsToWarrantsAxis" xlink:label="loc_galt_ModificationsToWarrantsAxis_321680" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_320932" xlink:to="loc_galt_ModificationsToWarrantsAxis_321680" use="optional" order="23" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_ModificationsToWarrantsDomain" xlink:label="loc_galt_ModificationsToWarrantsDomain_321681" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_galt_ModificationsToWarrantsAxis_321680" xlink:to="loc_galt_ModificationsToWarrantsDomain_321681" use="optional" order="24" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_ModificationsToWarrantsDomain" xlink:label="loc_galt_ModificationsToWarrantsDomain_321681_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_galt_ModificationsToWarrantsAxis_321680" xlink:to="loc_galt_ModificationsToWarrantsDomain_321681_default" use="optional" order="25" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_BeforeModificationOfTermsOfWarrantsMember" xlink:label="loc_galt_BeforeModificationOfTermsOfWarrantsMember_321682" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_galt_ModificationsToWarrantsDomain_321681" xlink:to="loc_galt_BeforeModificationOfTermsOfWarrantsMember_321682" use="optional" order="26" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_AfterModificationOfTermsOfWarrantsMember" xlink:label="loc_galt_AfterModificationOfTermsOfWarrantsMember_321683" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_galt_ModificationsToWarrantsDomain_321681" xlink:to="loc_galt_AfterModificationOfTermsOfWarrantsMember_321683" use="optional" order="27" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_LegalEntityAxis" xlink:label="loc_dei_LegalEntityAxis_321684" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_320932" xlink:to="loc_dei_LegalEntityAxis_321684" use="optional" order="28" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_321685" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_321684" xlink:to="loc_dei_EntityDomain_321685" use="optional" order="29" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2018/dei-2018-01-31.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_321685_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_321684" xlink:to="loc_dei_EntityDomain_321685_default" use="optional" order="30" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_TenXFundLPMember" xlink:label="loc_galt_TenXFundLPMember_321686" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityDomain_321685" xlink:to="loc_galt_TenXFundLPMember_321686" use="optional" order="31" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ConversionOfStockSharesConverted1" xlink:label="loc_us-gaap_ConversionOfStockSharesConverted1_321644" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_320935" xlink:to="loc_us-gaap_ConversionOfStockSharesConverted1_321644" use="optional" order="32" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ConversionOfStockSharesIssued1" xlink:label="loc_us-gaap_ConversionOfStockSharesIssued1_321645" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_320935" xlink:to="loc_us-gaap_ConversionOfStockSharesIssued1_321645" use="optional" order="33" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ClassOfWarrantOrRightNumberOfSecuritiesCalledByWarrantsOrRights" xlink:label="loc_us-gaap_ClassOfWarrantOrRightNumberOfSecuritiesCalledByWarrantsOrRights_321646" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_320935" xlink:to="loc_us-gaap_ClassOfWarrantOrRightNumberOfSecuritiesCalledByWarrantsOrRights_321646" use="optional" order="34" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_NonCashExpenseRelatedToExtensionOfWarrants" xlink:label="loc_galt_NonCashExpenseRelatedToExtensionOfWarrants_321647" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_320935" xlink:to="loc_galt_NonCashExpenseRelatedToExtensionOfWarrants_321647" use="optional" order="35" />
<loc xlink:type="locator" xlink:href="galt-20181231.xsd#galt_NumberOfWarrantsIssued" xlink:label="loc_galt_NumberOfWarrantsIssued_321648" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_320935" xlink:to="loc_galt_NumberOfWarrantsIssued_321648" use="optional" order="36" />
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_DerivativeLiabilityMeasurementInput" xlink:label="loc_us-gaap_DerivativeLiabilityMeasurementInput_321649" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_320935" xlink:to="loc_us-gaap_DerivativeLiabilityMeasurementInput_321649" use="optional" order="37" />
</definitionLink>
</linkbase>




