EXHIBIT 32.1

 WRITTEN STATEMENT OF CHIEF EXECUTIVE OFFICER PURSUANT TO SECTION 906 OF
                         THE SARBANES-OXLEY ACT OF 2002

The undersigned, the Chief Executive Officer of S1 Corporation (the "Company"),
hereby certifies that, to his knowledge on the date hereof:

(a)   the Form 10-K Report of the Company for the annual period ended December
      31, 2005 filed on the date hereof with the Securities and Exchange
      Commission (the "Report") fully complies with the requirements of Section
      13(a) or 15(d) of the Securities Exchange Act of 1934; and

(b)   information contained in the Report fairly presents, in all material
      respects, the financial condition and results of operations of the
      Company.

                                                   /s/ JAMES S. MAHAN, III
                                                   ----------------------------
                                                   James S. Mahan, III
                                                   Chief Executive Officer

A signed original of this written statement required by Section 906, or other
document authenticating, acknowledging, or otherwise adopting the signature that
appears in typed form within the electronic version of this written statement
required by Section 906, has been provided to the Company and will be retained
by the Company and furnished to the Securities and Exchange commission or its
staff upon request.

March 15, 2006