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

    <schemaVersion>X0304</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2012-01-30</periodOfReport>

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

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001314720</rptOwnerCik>
            <rptOwnerName>Matteson Timothy J</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>SVP and General Counsel</officerTitle>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2012-01-30</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>4389</value>
                    <footnoteId id="F1"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>10.04</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>18076</value>
                    <footnoteId id="F2"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <footnotes>
        <footnote id="F1">Reference is made to shares of restricted stock awarded under the Lakeland Bancorp Inc. 2009 Equity Compensation Program (&quot;Plan&quot;).  Subject to vesting and transferability requirements and restrictions under the Emergency Economic Stabilization Act of 2008 (&quot;EESA&quot;), as amended by the American Recovery and Reinvestment Act of 2009 (&quot;ARRA&quot;), the Interim Final Rule promulgated by the U.S. Department of Treasury on June 15, 2009, as applicable and as may be further amended, and the  Plan, the shares of the restricted stock shall vest as follows: 45% on January 30, 2014, 45% on January 30, 2015, with the remaining 10% on the later of January 30, 2017 or the date on which the Company repays the Capital Purchase Program investment.</footnote>
        <footnote id="F2">Includes 14,141 shares subject to restricted stock award that has not yet vested.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>Harry Cooper POA</signatureName>
        <signatureDate>2012-01-30</signatureDate>
    </ownerSignature>
</ownershipDocument>
