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

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2021-04-22</periodOfReport>

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

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001443274</rptOwnerCik>
            <rptOwnerName>Boigegrain Barbara A</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>8750 WEST BRYN MAWR AVENUE</rptOwnerStreet1>
            <rptOwnerStreet2>SUITE 1300</rptOwnerStreet2>
            <rptOwnerCity>CHICAGO</rptOwnerCity>
            <rptOwnerState>IL</rptOwnerState>
            <rptOwnerZipCode>60631</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2021-04-22</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness></transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>656</value>
                    <footnoteId id="F1"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>20.945</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>44401.1697</value>
                    <footnoteId id="F2"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <footnotes>
        <footnote id="F1">Award of stock granted pursuant to the Amended and Restated First Midwest Bancorp, Inc. Non-Employee Director Stock Plan. This award represents the first payment for 2021 of the equity portion of the Reporting Person's annual director compensation, which is paid in equal installments on a quarterly basis and may be paid, at the Reporting Person's option, in cash or stock. The number of shares represented by this award was determined by dividing the dollar value of the award by the price shown, which is the average high and low sale price of First Midwest Bancorp, Inc.'s common stock on the date of grant.</footnote>
        <footnote id="F2">Between January 12, 2021 and April 12, 2021, the Reporting Person acquired 249.4492 shares of First Midwest Bancorp, Inc. common stock pursuant to the First Midwest Bancorp, Inc. Dividend Reinvestment Plan. The information in this report is based upon a statement dated April 12, 2021.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>Debi Rouse, Attorney-in-fact for Barbara A. Boigegrain</signatureName>
        <signatureDate>2021-04-26</signatureDate>
    </ownerSignature>
</ownershipDocument>
