<?xml version="1.0"?>
<ownershipDocument>
    <schemaVersion>X0201</schemaVersion>

    <documentType>4</documentType>
<periodOfReport>2003-09-30</periodOfReport>
<notSubjectToSection16>0</notSubjectToSection16>
    <issuer>
        <issuerCik>0000703970</issuerCik>
        <issuerName>ALLEGIANT BANCORP INC/MO/</issuerName>
        <issuerTradingSymbol>ALLE</issuerTradingSymbol>
</issuer>
<reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001247099</rptOwnerCik>
            <rptOwnerName>WALLACE ROBERT E JR</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>10401 CLAYTON ROAD</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>ST LOUIS</rptOwnerCity>
            <rptOwnerState>MO</rptOwnerState>
            <rptOwnerZipCode>63131</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
            <reportingOwnerRelationship>
    <isDirector>1</isDirector>
    <isOfficer>0</isOfficer>
    <isTenPercentOwner>0</isTenPercentOwner>
    <isOther>0</isOther>
    <officerTitle></officerTitle>
    <otherText></otherText>
  </reportingOwnerRelationship>
</reportingOwner>
<nonDerivativeTable>
  <nonDerivativeTransaction>
    <securityTitle>
      <value>Common Stock</value>
      <footnoteId id="F1"/>
    </securityTitle>
    <transactionDate>
      <value>2003-09-30</value>
    </transactionDate>
    <transactionCoding>
      <transactionFormType>4</transactionFormType>
      <transactionCode>P</transactionCode>
      <equitySwapInvolved>0</equitySwapInvolved>
    </transactionCoding>
    <transactionTimeliness>
      <value></value>
    </transactionTimeliness>
    <transactionAmounts>
      <transactionShares>
        <value>323.4337</value>
      </transactionShares>
      <transactionPricePerShare>
        <value>20.29</value>
      </transactionPricePerShare>
      <transactionAcquiredDisposedCode>
        <value>A</value>
      </transactionAcquiredDisposedCode>
    </transactionAmounts>
    <postTransactionAmounts>
      <sharesOwnedFollowingTransaction>
        <value>9204.2412</value>
      </sharesOwnedFollowingTransaction>
    </postTransactionAmounts>
    <ownershipNature>
      <directOrIndirectOwnership>
        <value>I</value>
      </directOrIndirectOwnership>
      <natureOfOwnership>
        <value>Held jointly with spouse</value>
      </natureOfOwnership>
    </ownershipNature>
  </nonDerivativeTransaction>
</nonDerivativeTable>
<footnotes>
  <footnote id="F1">Directors fees converted to stock through the Dividend Reinvestment Plan</footnote>
</footnotes>
<ownerSignature>
  <signatureName>Robert E. Wallace</signatureName>
  <signatureDate>2003-10-02</signatureDate>
</ownerSignature>
</ownershipDocument>

