<Page>

                                  UNITED STATES
                        SECURITIES AND EXCHANGE COMMISSION
                             WASHINGTON, D.C. 20549



                                  SCHEDULE 13G

                   UNDER THE SECURITIES EXCHANGE ACT OF 1934
                               (AMENDMENT NO. 1)*


                              Unisys Corporation
--------------------------------------------------------------------------------
                                (Name of Issuer)

                                 Common Stock
--------------------------------------------------------------------------------
                        (Title of Class of Securities)

                                 909214-10-8
            --------------------------------------------------------
                                (CUSIP Number)

                               December 31, 2004
--------------------------------------------------------------------------------
            (Date of Event Which Requires Filing of this Statement)
                                          

Check the appropriate box to designate the rule pursuant to which this Schedule
is filed:

   /X/       Rule 13d-1(b)
   / /       Rule 13d-1(c)
   / /       Rule 13d-1(d)


*The remainder of this cover page shall be filled out for a reporting 
person's initial filing on this form with respect to the subject class of 
securities, and for any subsequent amendment containing information which 
would alter the disclosures provided in a prior cover page.

The information required in the remainder of this cover page shall not be 
deemed to be "filed" for the purpose of Section 18 of the Securities Exchange 
Act of 1934 ("Act") or otherwise subject to the liabilities of that section 
of the Act but shall be subject to all other provisions of the Act (however, 
see the Notes).


                                      Page 1
                                    of 7 Pages

<Page>

                                     SCHEDULE 13G

CUSIP NO. 909214-10-8                                       Page 2 of 7 Pages


-------------------------------------------------------------------------------
 (1) NAMES OF REPORTING PERSONS
     I.R.S. IDENTIFICATION NOS. OF ABOVE PERSONS (ENTITIES ONLY)


     Citigroup Global Markets Holdings Inc.
-------------------------------------------------------------------------------
 (2) CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP (SEE INSTRUCTIONS)

                                                                       (a)  / /
                                                                       (b)  / /
-------------------------------------------------------------------------------
 (3) SEC USE ONLY



-------------------------------------------------------------------------------
 (4) CITIZENSHIP OR PLACE OF ORGANIZATION                              New York


-------------------------------------------------------------------------------
       NUMBER OF           (5) SOLE VOTING POWER                              0

        SHARES                                                                
                           ----------------------------------------------------
     BENEFICIALLY          (6) SHARED VOTING POWER                   9,490,207*
                                                                             **
       OWNED BY
                           ----------------------------------------------------
        EACH               (7) SOLE DISPOSITIVE POWER                         0

      REPORTING                                                               
                           ----------------------------------------------------
       PERSON              (8) SHARED DISPOSITIVE POWER              9,490,207*
                                                                             **
        WITH:
-------------------------------------------------------------------------------
(9) AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON     9,490,207*
                                                                             **

-------------------------------------------------------------------------------
(10) CHECK IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES (SEE 
     INSTRUCTIONS)  / /


-------------------------------------------------------------------------------
(11) PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (9)                    2.8%*
                                                                             **

-------------------------------------------------------------------------------
(12) TYPE OF REPORTING PERSON (SEE INSTRUCTIONS)                             HC



-------------------------------------------------------------------------------
*  Assumes conversion/exercise of certain securities held.
** Includes shares for which the reporting person disclaims beneficial 
   ownership. See Item 4(a).

<Page>

                                     SCHEDULE 13G

CUSIP NO. 909214-10-8                                        Page 3 of 7 Pages


-------------------------------------------------------------------------------
 (1) NAMES OF REPORTING PERSONS
     I.R.S. IDENTIFICATION NOS. OF ABOVE PERSONS (ENTITIES ONLY)


     Citigroup Inc.
-------------------------------------------------------------------------------
 (2) CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP (SEE INSTRUCTIONS)

                                                                       (a)  / /
                                                                       (b)  / /
-------------------------------------------------------------------------------
 (3) SEC USE ONLY



-------------------------------------------------------------------------------
 (4) CITIZENSHIP OR PLACE OF ORGANIZATION                              Delaware


-------------------------------------------------------------------------------
       NUMBER OF           (5) SOLE VOTING POWER                              0

        SHARES                                                                
                           ----------------------------------------------------
     BENEFICIALLY          (6) SHARED VOTING POWER                   9,671,429*
                                                                             **
                                                                            ***
       OWNED BY
                           ----------------------------------------------------
        EACH               (7) SOLE DISPOSITIVE POWER                         0

      REPORTING                                                               
                           ----------------------------------------------------
       PERSON              (8) SHARED DISPOSITIVE POWER              9,671,429*
                                                                             **
                                                                            ***

        WITH:
-------------------------------------------------------------------------------
(9) AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON     9,671,429*
                                                                             **
                                                                            ***

-------------------------------------------------------------------------------
(10) CHECK IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES (SEE 
     INSTRUCTIONS)  / /


-------------------------------------------------------------------------------
(11) PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (9)                    2.9%*
                                                                             **
                                                                            ***


-------------------------------------------------------------------------------
(12) TYPE OF REPORTING PERSON (SEE INSTRUCTIONS)                             HC


-------------------------------------------------------------------------------
*   Assumes conversion/exercise of certain securities held.
**  Includes shares for which the reporting person disclaims beneficial 
    ownership. See Item 4(a).
*** Includes shares held by the other reporting person.

<Page>

Item 1(a).     Name of Issuer:

               Unisys Corporation

Item 1(b).     Address of Issuer's Principal Executive Offices:

               Unisys Way
               Blue Bell, PA 19424

Item 2(a).     Name of Person Filing:

               Citigroup Global Markets Holdings Inc. ("CGM Holdings")
               Citigroup Inc. ("Citigroup")

Item 2(b).     Address of Principal Office or, if none, Residence:

               The address of the principal office of CGM Holdings is:

               388 Greenwich Street
               New York, NY 10013

               The address of the principal office of Citigroup is:

               399 Park Avenue
               New York, NY 10043

Item 2(c).     Citizenship or Place of Organization:

               CGM Holdings is a New York corporation. 

               Citigroup is a Delaware corporation.

Item 2(d).     Title of Class of Securities:

               Common Stock

Item 2(e).     CUSIP Number:

               909214-10-8


                                       Page 4
                                     of 7 Pages
<Page>

Item 3.      If This Statement is Filed Pursuant to Sections 240.13d-1(b) or
             240.13d-2(b) or (c), Check Whether the Person Filing Is a(n):

             (a)  [ ] Broker or dealer registered under Section 15 of the 
                      Act (15 U.S.C. 78o);

             (b)  [ ] Bank as defined in Section 3(a)(6) of the Act 
                      (15 U.S.C. 78c);

             (c)  [ ] Insurance company as defined in Section 3(a)(19) of 
                      the Act (15 U.S.C. 78c);

             (d)  [ ] Investment company registered under Section 8 of the 
                      Investment Company Act of 1940 (15 U.S.C. 80a-8);

             (e)  [ ] Investment adviser in accordance with Section 
                      240.13d-1(b)(1)(ii)(E);

             (f)  [ ] Employee benefit plan or endowment fund in accordance 
                      with Section 240.13d-1(b)(1)(ii)(F);

             (g)  [X] Parent holding company or control person in accordance
                      with Section  240.13d-1(b)(1)(ii)(G); (See Exhibit 2)

             (h)  [ ] Savings association as defined in Section 3(b) of the 
                      Federal Deposit Insurance Act (12 U.S.C. 1813);

             (i)  [ ] Church plan that is excluded from the definition of an 
                      investment company under Section 3(c)(14) of the 
                      Investment Company Act of 1940 (15 U.S.C. 80a-3);

             (j)  [ ] Group, in accordance with Section 240.13d-1(b)(1)(ii)(J).


Item 4.   Ownership. (as of December 31, 2004)

          (a)  Amount beneficially owned: See item 9 of cover pages

               (Includes shares for which the reporting person disclaims 
               beneficial ownership.)

          (b)  Percent of Class: See item 11 of cover pages

          (c)  Number of shares as to which the person has:

                  (i)   Sole power to vote or to direct the vote:

                  (ii)  Shared power to vote or to direct the vote:

                  (iii) Sole power to dispose or to direct the disposition of:

                  (iv)  Shared power to dispose or to direct the disposition of:


                  See Items 5-8 of cover pages


                                       Page 5
                                     of 7 Pages
<Page>

Item 5.   Ownership of Five Percent or Less of a Class.

          If this statement is being filed to report the fact that as of the 
          date hereof the reporting person has ceased to be the beneficial owner
          of more than five percent of the class of securities, check the
          following [X].

Item 6.   Ownership of More Than Five Percent on Behalf of Another Person.

          Not Applicable.

Item 7.   Identification and Classification of the Subsidiary which Acquired 
          the Security being Reported on by the Parent Holding Company.

          See Exhibit 2 for the identity and classification of the 
          subsidiaries which directly beneficially own the securities 
          reported herein.

Item 8.   Identification and Classification of Members of the Group.

          Not Applicable.

Item 9.   Notice of Dissolution of Group. 

          Not Applicable.



                                       Page 6
                                     of 7 Pages

<Page>

Item 10.  Certification.

          By signing below I certify that, to the best of my knowledge and 
belief, the securities referred to above were acquired and are held in the 
ordinary course of business and were not acquired and are not held for the 
purpose of or with the effect of changing or influencing the control of the 
issuer of the securities and were not acquired and are not held in connection 
with or as a participant in any transaction having that purpose or effect.

SIGNATURE.

          After reasonable inquiry and to the best of my knowledge and 
belief, I certify that the information set forth in this statement is true, 
complete and correct.

Dated: February 8, 2005


                      CITIGROUP GLOBAL MARKETS HOLDINGS INC.



                      By: /s/ Serena D. Moe
                         --------------------------------------------
                         Name:  Serena D. Moe
                         Title: Assistant Secretary



                      CITIGROUP INC.



                      By: /s/ Serena D. Moe
                         --------------------------------------------
                         Name:  Serena D. Moe
                         Title: Assistant Secretary


                                 Page 7
                               of 7 Pages

<Page>


                          EXHIBIT INDEX TO SCHEDULE 13G
                          -----------------------------



EXHIBIT 1
---------

Agreement between CGM Holdings and Citigroup as to joint filing of Schedule 13G

EXHIBIT 2
---------

Identification and Item 3 classification of the subsidiaries which acquired 
the securities being reported by the parent holding company.




<Page>


                                    EXHIBIT 1
                                    ---------

                  AGREEMENT AS TO JOINT FILING OF SCHEDULE 13G
                  --------------------------------------------

Each of the undersigned hereby affirms that it is individually eligible to use
Schedule 13G, and agrees that this Schedule 13G is filed on its behalf.

Dated: February 8, 2005


                      CITIGROUP GLOBAL MARKETS HOLDINGS INC.



                      By: /s/ Serena D. Moe
                         --------------------------------------------
                         Name:  Serena D. Moe
                         Title: Assistant Secretary



                      CITIGROUP INC.



                      By: /s/ Serena D. Moe
                         --------------------------------------------
                         Name:  Serena D. Moe
                         Title: Assistant Secretary





<Page>



<Table>
<Caption>

                                              EXHIBIT 2
                                              ---------

<S>                    <C>
                      IDENTIFICATION AND CLASSIFICATION OF SUBSIDIARIES WHICH ACQUIRED SECURITIES
                      ----------------------------------------------------------------------------


                      Citigroup Global Markets Inc. is a broker or dealer registered under Section 15
                                           of the Act. (15 U.S.C. 78o).

                      Smith Barney Fund Management LLC is an investment adviser in accordance with
                                        Section 240.13d -1(b) (1)(ii)(E).


                         Each of the undersigned hereby affirms the identification and Item 3
                        classification of the subsidiaries which acquired the securities filed 
                                           for in this Schedule 13G.



                    Date: February 8, 2005


                                          CITIGROUP GLOBAL MARKETS HOLDINGS INC.


                                          By: /s/ Serena D. Moe
                                              --------------------------------
                                              Name:  Serena D. Moe
                                              Title: Assistant Secretary  


                                          CITIGROUP INC.


                                          By: /s/ Serena D. Moe
                                              --------------------------------
                                               Name:  Serena D. Moe
                                               Title: Assistant Secretary

</Table>