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

    <documentType>4</documentType><periodOfReport>2003-06-06</periodOfReport>
    <issuer>
        <issuerCik>0001126328</issuerCik>
        <issuerName>PRINCIPAL FINANCIAL GROUP INC</issuerName>
        <issuerTradingSymbol>PFG</issuerTradingSymbol></issuer><reportingOwner><reportingOwnerId>
            <rptOwnerCik>0001166872</rptOwnerCik>
            <rptOwnerName>SHAFF KAREN E</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>Sr. VP and General Counsel</officerTitle></reportingOwnerRelationship></reportingOwner><derivativeSecurity><securityTitle><value>Phantom Stock Units</value></securityTitle><conversionOrExercisePrice><value>0</value><footnoteId id="F1"/></conversionOrExercisePrice><transactionDate><value>2003-06-06</value></transactionDate><transactionCoding><transactionFormType>4</transactionFormType><transactionCode>A</transactionCode><equitySwapInvolved>0</equitySwapInvolved><footnoteId id="F2"/></transactionCoding><transactionAmounts><transactionShares><value>43.18</value></transactionShares><transactionValue><value>32.87</value></transactionValue><transactionAcquiredDisposedCode><value>A</value></transactionAcquiredDisposedCode></transactionAmounts><exerciseDate><value>1988-08-08</value><footnoteId id="F3"/></exerciseDate><expirationDate><value>1988-08-08</value><footnoteId id="F3"/></expirationDate><underlyingSecurity><underlyingSecurityTitle><value>Common Stock</value></underlyingSecurityTitle><underlyingSecurityShares><value>43.18</value></underlyingSecurityShares></underlyingSecurity><postTransactionAmounts><sharesOwnedFollowingTransaction><value>1428.71</value></sharesOwnedFollowingTransaction></postTransactionAmounts><ownershipNature><directOrIndirectOwnership><value>D</value></directOrIndirectOwnership><natureOfOwnership><value></value></natureOfOwnership></ownershipNature></derivativeSecurity><footnotes><footnote id="F1">Security converts to common stock on a one-for-one basis.</footnote><footnote id="F2">Pursuant to 10b5-1 plan adopted March 25, 2002.</footnote><footnote id="F3">The reported phantom stock units were acquired pursuant to the Principal Select Savings Excess Plan and may be transferred at any time into another investment alternative under the plan.  Interests under the plan will be settled upon the reporting person's retirement or other termination of service.</footnote></footnotes><ownerSignature><signatureName>Joyce N. Hoffman, by Power of Attorney</signatureName><signatureDate>2003-06-10</signatureDate></ownerSignature></ownershipDocument>

