<?xml version="1.0"?>
<ownershipDocument>
    <schemaVersion>X0101</schemaVersion>

<documentType>3</documentType><periodOfReport>2003-05-05</periodOfReport>
    <noSecuritiesOwned>0</noSecuritiesOwned>
<issuer><issuerCik>0001109357</issuerCik>
        <issuerName>EXELON CORP</issuerName>
<issuerTradingSymbol>EXC</issuerTradingSymbol></issuer><reportingOwner><reportingOwnerId><rptOwnerCik>0001230821</rptOwnerCik>
            <rptOwnerName>BEMIS MICHAEL B</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>  </rptOwnerStreet1>
            <rptOwnerStreet2>  </rptOwnerStreet2>
            <rptOwnerCity>  </rptOwnerCity>
            <rptOwnerState>  </rptOwnerState>
            <rptOwnerZipCode>  </rptOwnerZipCode>
        </reportingOwnerAddress>
<reportingOwnerRelationship>
<isDirector>0</isDirector><isOfficer>1</isOfficer><isTenPercentOwner>0</isTenPercentOwner><isOther>0</isOther><officerTitle>Senior Vice President</officerTitle></reportingOwnerRelationship></reportingOwner><nonDerivativeSecurity><securityTitle><value>Common Stock</value></securityTitle><transactionAmounts><transactionShares><value>6773</value></transactionShares></transactionAmounts><ownershipNature><directOrIndirectOwnership><value>D</value></directOrIndirectOwnership><natureOfOwnership><value></value></natureOfOwnership></ownershipNature></nonDerivativeSecurity><derivativeSecurity><securityTitle><value>Deferred Comp. Phantom Shares</value></securityTitle><conversionOrExercisePrice><value>0</value><footnoteId id="F1"/></conversionOrExercisePrice><exerciseDate><value>1988-08-08</value></exerciseDate><expirationDate><value>1988-08-08</value></expirationDate><underlyingSecurity><underlyingSecurityTitle><value>Common Stock</value></underlyingSecurityTitle><underlyingSecurityShares><value>127</value></underlyingSecurityShares></underlyingSecurity><ownershipNature><directOrIndirectOwnership><value>D</value></directOrIndirectOwnership><natureOfOwnership><value></value></natureOfOwnership></ownershipNature></derivativeSecurity><footnotes><footnote id="F1">Phantom shares held in a multi-fund deferred compensation account to be settled for cash on a 1:1 basis upon the reporting person's termination of employment for any reason.  Shares are acquired through regular periodic payroll contributions and the reinvestment of dividends</footnote></footnotes><ownerSignature><signatureName>Michael B. Bemis</signatureName><signatureDate>2003-05-07</signatureDate></ownerSignature></ownershipDocument>

