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

    <schemaVersion>X0303</schemaVersion>

    <documentType>4/A</documentType>

    <periodOfReport>2009-05-20</periodOfReport>

    <dateOfOriginalSubmission>2009-05-22</dateOfOriginalSubmission>

    <issuer>
        <issuerCik>0000702325</issuerCik>
        <issuerName>FIRST MIDWEST BANCORP INC</issuerName>
        <issuerTradingSymbol>FMBI</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001452076</rptOwnerCik>
            <rptOwnerName>Carapella Victor P</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>ONE PIERCE PLACE</rptOwnerStreet1>
            <rptOwnerStreet2>SUITE 1500</rptOwnerStreet2>
            <rptOwnerCity>ITASCA</rptOwnerCity>
            <rptOwnerState>IL</rptOwnerState>
            <rptOwnerZipCode>60143</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isOfficer>1</isOfficer>
            <officerTitle>EVP, Comm'l Banking Group Mgr.</officerTitle>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
                <footnoteId id="F1"/>
                <footnoteId id="F2"/>
            </securityTitle>
            <transactionDate>
                <value>2009-05-20</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness></transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>18797</value>
                    <footnoteId id="F3"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>33988</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <footnotes>
        <footnote id="F1">This Form 4 is being amended to correct an administrative error in the number of shares awarded to the Reporting Person as 18,797 (rather than 15,561) shares and the disclosure contained in Footnote 3.</footnote>
        <footnote id="F2">Long Term Incentive Compensation Restricted Stock Awards (RSAs) granted pursuant to the Restated First Midwest Bancorp, Inc. Omnibus Stock and Incentive Plan.</footnote>
        <footnote id="F3">The number of shares represented by this award was determined by dividing the dollar value of the award granted to the reporting person by $9.27 (the average high and low sale price of one share of the company's common stock on the date of grant).  This award will vest in two equal installments, specifically on the second and third anniversaries of the date of grant.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>Andrea L. Stangl, Attorney-in-fact for Victor P. Carapella</signatureName>
        <signatureDate>2011-05-24</signatureDate>
    </ownerSignature>
</ownershipDocument>
