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

    <documentType>4</documentType><periodOfReport>2003-07-09</periodOfReport>
    <issuer>
        <issuerCik>0001042064</issuerCik>
        <issuerName>NORTHWEST BANCORP INC</issuerName>
        <issuerTradingSymbol>NWSB</issuerTradingSymbol></issuer><reportingOwner><reportingOwnerId>
            <rptOwnerCik>0001186990</rptOwnerCik>
            <rptOwnerName>HANNA JOHN O</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>  </rptOwnerStreet1>
            <rptOwnerStreet2>  </rptOwnerStreet2>
            <rptOwnerCity>  </rptOwnerCity>
            <rptOwnerState>  </rptOwnerState>
            <rptOwnerZipCode>  </rptOwnerZipCode>
        </reportingOwnerAddress>
            <reportingOwnerRelationship>
<isDirector>1</isDirector><isOfficer>1</isOfficer><isTenPercentOwner>0</isTenPercentOwner><isOther>0</isOther><officerTitle>Chairman of the Board</officerTitle></reportingOwnerRelationship></reportingOwner><nonDerivativeSecurity><securityTitle><value>Northwest Bancorp, Inc.</value></securityTitle><transactionDate><value>2003-07-09</value></transactionDate><deemedExecutionDate><value>2003-07-09</value></deemedExecutionDate><transactionCoding><transactionFormType>4</transactionFormType><transactionCode>G</transactionCode><equitySwapInvolved>0</equitySwapInvolved><footnoteId id="F1"/></transactionCoding><transactionAmounts><transactionShares><value>600</value></transactionShares><transactionValue><value>17.105</value></transactionValue><transactionAcquiredDisposedCode><value>D</value></transactionAcquiredDisposedCode></transactionAmounts><postTransactionAmounts><sharesOwnedFollowingTransaction><value>203535</value></sharesOwnedFollowingTransaction></postTransactionAmounts><ownershipNature><directOrIndirectOwnership><value>D</value></directOrIndirectOwnership><natureOfOwnership><value></value></natureOfOwnership></ownershipNature></nonDerivativeSecurity><footnotes><footnote id="F1">600 shares of Northwest Bancorp, Inc. stock were gifted to Warren General Hospital Cancer Center.</footnote></footnotes><ownerSignature><signatureName>John O. Hanna</signatureName><signatureDate>2003-07-10</signatureDate></ownerSignature></ownershipDocument>

