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

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2014-02-19</periodOfReport>

    <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, Dir. of Comm'l Banking</officerTitle>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
                <footnoteId id="F1"/>
            </securityTitle>
            <transactionDate>
                <value>2014-02-19</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness></transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>10286</value>
                    <footnoteId id="F2"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                    <footnoteId id="F2"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>94604</value>
                    <footnoteId id="F3"/>
                    <footnoteId id="F4"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>49880</value>
                    <footnoteId id="F5"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>By NQ Stock Option Gain Deferral Plan</value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>26009</value>
                    <footnoteId id="F6"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>By Profit Sharing Plan</value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
    </nonDerivativeTable>

    <footnotes>
        <footnote id="F1">Long Term Incentive Compensation restricted stock units granted pursuant to the Amended and Restated First Midwest Bancorp, Inc. Omnibus Stock and Incentive Plan.</footnote>
        <footnote id="F2">The number of units represented by this award was determined by dividing the dollar value of the award granted to the Reporting Person by $16.078 (the average high and low sale price of one share of the Issuer'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>
        <footnote id="F3">Between February 27, 2013 and February 21, 2014, the Reporting Person acquired 70 shares of First Midwest Bancorp, Inc. Common Stock under the First Midwest Bancorp, Inc. Dividend Reinvestment Plan. The information in this report is based on a statement dated January 14, 2014.</footnote>
        <footnote id="F4">39,174 of these shares are restricted stock units.</footnote>
        <footnote id="F5">Between February 27, 2013 and February 21, 2014, the Reporting Person acquired 503 shares of First Midwest Bancorp, Inc. Common Stock pursuant to the Dividend Reinvestment feature under the First Midwest Bancorp, Inc. Non-Qualified Gain Deferral Plan. The information in this report is based on a plan statement dated January 14, 2014.</footnote>
        <footnote id="F6">Between February 27, 2013 and February 21, 2014, the Reporting Person acquired 261 shares of First Midwest Bancorp, Inc. common stock under the First Midwest Bancorp, Inc. Savings and Profit Sharing Plan pursuant to the Dividend Reinvestment feature of the Plan and 38 shares were sold from the Reporting Person's Plan account in satisfaction of quarterly Plan administration fees. This information is based on a Plan statement dated January 31, 2014.</footnote>
    </footnotes>

    <remarks>Exhibit 24 - Power of Attorney</remarks>

    <ownerSignature>
        <signatureName>Andrea L. Stangl, Attorney-in-fact for Victor P. Carapella</signatureName>
        <signatureDate>2014-02-21</signatureDate>
    </ownerSignature>
</ownershipDocument>
