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

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2017-10-26</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>ONE PIERCE PLACE</rptOwnerStreet1>
            <rptOwnerStreet2>SUITE 1500</rptOwnerStreet2>
            <rptOwnerCity>ITASCA</rptOwnerCity>
            <rptOwnerState>IL</rptOwnerState>
            <rptOwnerZipCode>60143</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2017-10-26</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness></transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>628</value>
                    <footnoteId id="F1"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>22.90</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>32332</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 third payment for 2017 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 granted to the Reporting Person by the price shown, which is the average high and low sale price of one share of the Issuer's Common Stock on the date of grant.</footnote>
        <footnote id="F2">Between July 12, 2017 and October 10, 2017, the Reporting Person acquired 116 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 October 10, 2017.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>Andrea L. Stangl, Attorney-in-fact for Barbara A. Boigegrain</signatureName>
        <signatureDate>2017-10-30</signatureDate>
    </ownerSignature>
</ownershipDocument>
