<?xml version="1.0"?>
<ownershipDocument>
    <schemaVersion>X0201</schemaVersion>

    <documentType>4</documentType><periodOfReport>2003-10-01</periodOfReport>
    <issuer>
        <issuerCik>0000054507</issuerCik>
        <issuerName>WESTAR ENERGY INC /KS</issuerName>
        <issuerTradingSymbol>WR</issuerTradingSymbol></issuer><reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001184809</rptOwnerCik>
            <rptOwnerName>NETTELS JOHN C JR</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>818 S. KANSAS AVE.</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>TOPEKA</rptOwnerCity>
            <rptOwnerState>KS</rptOwnerState>
            <rptOwnerZipCode>66612</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
            <reportingOwnerRelationship>
<isDirector>1</isDirector><isOfficer>0</isOfficer><isTenPercentOwner>0</isTenPercentOwner><isOther>0</isOther></reportingOwnerRelationship></reportingOwner><nonDerivativeTable><nonDerivativeHolding><securityTitle><value>Common Stock, Par Value $5.00</value></securityTitle><postTransactionAmounts><sharesOwnedFollowingTransaction><value>9740</value><footnoteId id="F1"/></sharesOwnedFollowingTransaction></postTransactionAmounts><ownershipNature><directOrIndirectOwnership><value>D</value></directOrIndirectOwnership><natureOfOwnership><value/></natureOfOwnership></ownershipNature></nonDerivativeHolding><nonDerivativeHolding><securityTitle><value>Common Stock, Par Value $5.00</value></securityTitle><postTransactionAmounts><sharesOwnedFollowingTransaction><value>500</value></sharesOwnedFollowingTransaction></postTransactionAmounts><ownershipNature><directOrIndirectOwnership><value>I</value></directOrIndirectOwnership><natureOfOwnership><value>By trust</value><footnoteId id="F2"/></natureOfOwnership></ownershipNature></nonDerivativeHolding></nonDerivativeTable><derivativeTable><derivativeTransaction><securityTitle><value>Stock Units</value></securityTitle><conversionOrExercisePrice><value>0</value><footnoteId id="F3"/></conversionOrExercisePrice><transactionDate><value>2003-10-01</value><footnoteId id="F4"/></transactionDate><transactionCoding><transactionFormType>4</transactionFormType><transactionCode>A</transactionCode><equitySwapInvolved>0</equitySwapInvolved></transactionCoding><transactionAmounts><transactionShares><value>214</value></transactionShares><transactionPricePerShare><value>0</value></transactionPricePerShare><transactionAcquiredDisposedCode><value>A</value></transactionAcquiredDisposedCode></transactionAmounts><exerciseDate><footnoteId id="F4"/></exerciseDate><expirationDate><footnoteId id="F4"/></expirationDate><underlyingSecurity><underlyingSecurityTitle><value>Common Stock</value></underlyingSecurityTitle><underlyingSecurityShares><value>214</value></underlyingSecurityShares></underlyingSecurity><postTransactionAmounts><sharesOwnedFollowingTransaction><value>3619</value></sharesOwnedFollowingTransaction></postTransactionAmounts><ownershipNature><directOrIndirectOwnership><value>D</value></directOrIndirectOwnership><natureOfOwnership><value/></natureOfOwnership></ownershipNature></derivativeTransaction></derivativeTable><footnotes><footnote id="F1">Includes 95 shares acquired through the reinvestment of dividends in October 2003.  </footnote><footnote id="F2">Shares held in trust of which the reporting person is a co-trustee with shared investment and voting power.</footnote><footnote id="F3">Units convert into common stock on a one for one basis.</footnote><footnote id="F4">Share units granted pursuant to a deferred director compensation plan and payable in stock.</footnote></footnotes><remarks/><ownerSignature><signatureName>Cynthia S. Couch on behalf of John C. Nettels, Jr. by power of attorney</signatureName><signatureDate>2003-10-03</signatureDate></ownerSignature></ownershipDocument>

