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

    <documentType>4</documentType><periodOfReport>2003-08-20</periodOfReport>
    <issuer>
        <issuerCik>0001042064</issuerCik>
        <issuerName>NORTHWEST BANCORP INC</issuerName>
        <issuerTradingSymbol>NWSB</issuerTradingSymbol></issuer><reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001186998</rptOwnerCik>
            <rptOwnerName>LAROCCA GREGORY C</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>301 SECOND AVENUE</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>WARREN,</rptOwnerCity>
            <rptOwnerState>PA</rptOwnerState>
            <rptOwnerZipCode>16365</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
            <reportingOwnerRelationship>
<isDirector>0</isDirector><isOfficer>1</isOfficer><isTenPercentOwner>0</isTenPercentOwner><isOther>0</isOther><officerTitle>Exec. V.P. Admin/Invest. Serv.</officerTitle></reportingOwnerRelationship></reportingOwner><nonDerivativeTable><nonDerivativeHolding><securityTitle><value>Northwest Bancorp, Inc. Common Stock</value></securityTitle><postTransactionAmounts><sharesOwnedFollowingTransaction><value>40505.86</value></sharesOwnedFollowingTransaction></postTransactionAmounts><ownershipNature><directOrIndirectOwnership><value>D</value></directOrIndirectOwnership><natureOfOwnership><value/></natureOfOwnership></ownershipNature></nonDerivativeHolding><nonDerivativeHolding><securityTitle><value>Northwest Bancorp, Inc. Common Stock</value></securityTitle><postTransactionAmounts><sharesOwnedFollowingTransaction><value>1112</value></sharesOwnedFollowingTransaction></postTransactionAmounts><ownershipNature><directOrIndirectOwnership><value>I</value></directOrIndirectOwnership><natureOfOwnership><value>By IRA</value></natureOfOwnership></ownershipNature></nonDerivativeHolding><nonDerivativeHolding><securityTitle><value>Northwest Bancorp, Inc. Common Stock</value></securityTitle><postTransactionAmounts><sharesOwnedFollowingTransaction><value>11821.391</value></sharesOwnedFollowingTransaction></postTransactionAmounts><ownershipNature><directOrIndirectOwnership><value>I</value></directOrIndirectOwnership><natureOfOwnership><value>By 401-K</value></natureOfOwnership></ownershipNature></nonDerivativeHolding><nonDerivativeHolding><securityTitle><value>Northwest Bancorp, Inc. Common Stock</value></securityTitle><postTransactionAmounts><sharesOwnedFollowingTransaction><value>9445.1024</value></sharesOwnedFollowingTransaction></postTransactionAmounts><ownershipNature><directOrIndirectOwnership><value>I</value></directOrIndirectOwnership><natureOfOwnership><value>By ESOP</value></natureOfOwnership></ownershipNature></nonDerivativeHolding><nonDerivativeHolding><securityTitle><value>Northwest Bancorp, Inc. Common Stock</value></securityTitle><postTransactionAmounts><sharesOwnedFollowingTransaction><value>841</value></sharesOwnedFollowingTransaction></postTransactionAmounts><ownershipNature><directOrIndirectOwnership><value>I</value></directOrIndirectOwnership><natureOfOwnership><value>By Wife's IRA</value></natureOfOwnership></ownershipNature></nonDerivativeHolding></nonDerivativeTable><derivativeTable><derivativeHolding><securityTitle><value>Option Plan (Right to Buy)</value></securityTitle><conversionOrExercisePrice><value>5.875</value></conversionOrExercisePrice><exerciseDate><value>1995-12-20</value></exerciseDate><expirationDate><value>2005-12-20</value></expirationDate><underlyingSecurity><underlyingSecurityTitle><value>Northwest Bancorp, Inc.</value></underlyingSecurityTitle><underlyingSecurityShares><value>16667</value></underlyingSecurityShares></underlyingSecurity><postTransactionAmounts><sharesOwnedFollowingTransaction><value>16667</value></sharesOwnedFollowingTransaction></postTransactionAmounts><ownershipNature><directOrIndirectOwnership><value>D</value></directOrIndirectOwnership><natureOfOwnership><value/></natureOfOwnership></ownershipNature></derivativeHolding><derivativeHolding><securityTitle><value>Option Plan (Right to Buy)</value></securityTitle><conversionOrExercisePrice><value>9.78</value></conversionOrExercisePrice><exerciseDate><value>2001-10-18</value></exerciseDate><expirationDate><value>2011-10-17</value></expirationDate><underlyingSecurity><underlyingSecurityTitle><value>Northwest Bancorp, Inc.</value></underlyingSecurityTitle><underlyingSecurityShares><value>4300</value></underlyingSecurityShares></underlyingSecurity><postTransactionAmounts><sharesOwnedFollowingTransaction><value>4300</value></sharesOwnedFollowingTransaction></postTransactionAmounts><ownershipNature><directOrIndirectOwnership><value>D</value></directOrIndirectOwnership><natureOfOwnership><value/></natureOfOwnership></ownershipNature></derivativeHolding><derivativeHolding><securityTitle><value>Option Plan (Right to Buy)</value></securityTitle><conversionOrExercisePrice><value>13.302</value></conversionOrExercisePrice><exerciseDate><value>2002-08-21</value></exerciseDate><expirationDate><value>2012-08-21</value></expirationDate><underlyingSecurity><underlyingSecurityTitle><value>Northwest Bancorp, Inc.</value></underlyingSecurityTitle><underlyingSecurityShares><value>5100</value></underlyingSecurityShares></underlyingSecurity><postTransactionAmounts><sharesOwnedFollowingTransaction><value>5100</value></sharesOwnedFollowingTransaction></postTransactionAmounts><ownershipNature><directOrIndirectOwnership><value>D</value></directOrIndirectOwnership><natureOfOwnership><value/></natureOfOwnership></ownershipNature></derivativeHolding><derivativeTransaction><securityTitle><value>Option Plan (Right to Buy)</value></securityTitle><conversionOrExercisePrice><value>16.59</value></conversionOrExercisePrice><transactionDate><value>2003-08-20</value></transactionDate><transactionCoding><transactionFormType>4</transactionFormType><transactionCode>A</transactionCode><equitySwapInvolved>0</equitySwapInvolved></transactionCoding><transactionAmounts><transactionShares><value>5100</value><footnoteId id="F1"/></transactionShares><transactionPricePerShare><value>16.59</value></transactionPricePerShare><transactionAcquiredDisposedCode><value>A</value></transactionAcquiredDisposedCode></transactionAmounts><exerciseDate><value>2003-08-20</value></exerciseDate><expirationDate><value>2013-08-20</value></expirationDate><underlyingSecurity><underlyingSecurityTitle><value>Northwest Bancorp, Inc.</value></underlyingSecurityTitle><underlyingSecurityShares><value>5100</value></underlyingSecurityShares></underlyingSecurity><postTransactionAmounts><sharesOwnedFollowingTransaction><value>5100</value></sharesOwnedFollowingTransaction></postTransactionAmounts><ownershipNature><directOrIndirectOwnership><value>D</value></directOrIndirectOwnership><natureOfOwnership><value/></natureOfOwnership></ownershipNature></derivativeTransaction></derivativeTable><footnotes><footnote id="F1">A 20% increment of this option became exercisable on August 20, 2003. This option will continue to become exercisable each August thereafter in 20% increments through August 20, 2007. Although these options were granted by the Board on August 20, 2003 the insider was not issued an agreement with specific option grant information until November 6, 2003.</footnote></footnotes><remarks/><ownerSignature><signatureName>Gregory C. LaRocca</signatureName><signatureDate>2003-11-07</signatureDate></ownerSignature></ownershipDocument>

