<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0306</schemaVersion>

    <documentType>4/A</documentType>

    <periodOfReport>2017-02-22</periodOfReport>

    <dateOfOriginalSubmission>2017-02-24</dateOfOriginalSubmission>

    <issuer>
        <issuerCik>0000846901</issuerCik>
        <issuerName>LAKELAND BANCORP INC</issuerName>
        <issuerTradingSymbol>LBAI</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001487776</rptOwnerCik>
            <rptOwnerName>Nigro James M.</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O LAKELAND BANCORP, INC.</rptOwnerStreet1>
            <rptOwnerStreet2>250 OAK RIDGE ROAD</rptOwnerStreet2>
            <rptOwnerCity>OAK RIDGE</rptOwnerCity>
            <rptOwnerState>NJ</rptOwnerState>
            <rptOwnerZipCode>07438</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>1</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle>EVP Chief Risk Officer</officerTitle>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2017-02-22</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>3555</value>
                    <footnoteId id="F1"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>24408</value>
                    <footnoteId id="F2"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <footnotes>
        <footnote id="F1">Represents performance-based Restricted Stock Units (RSUs), each representing the right to receive, following vesting, one share of common stock.  The resulting shares acquired upon vesting of the RSUs is contingent upon the achievement of pre-established performance metrics over a 3 year period beginning 1-1-2017 and ending on 12-31-2019 (the &quot;Performance Period&quot;), provided that the Reporting Person remains employed by the Issuer on the date following the Performance Period that the Compensation Committee of the Issuer's Board of Directors determines that the performance goals for the final year of the Performance Period have been satisfied. If employment terminates sooner, the RSUs will be forfeited unless termination of employment occurs because of death or, subject to certain service requirements, disability or normal or early retirement, in any of which events the RSUs may vest in whole or in part.  The RSUs were granted under the Issuer's 2009 Equity Compensation Program.</footnote>
        <footnote id="F2">Represents a total of 11,055 RSU's (including those acquired on 2/22/17) that have not vested.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>Patricia Backman POA</signatureName>
        <signatureDate>2017-02-27</signatureDate>
    </ownerSignature>
</ownershipDocument>
