UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM 8-K

CURRENT REPORT

PURSUANT TO SECTION 13 OR 15(d) OF THE

SECURITIES EXCHANGE ACT OF 1934

Date of Report (Date of earliest event reported): January 19, 2012

S1 CORPORATION

(Exact name of registrant as specified in its charter)

Registrant’s telephone number, including area code: (404) 923-3500

Not Applicable

(Former name or former address, if changed since last report)

Check the appropriate box below if the Form 8-K filing is intended to simultaneously satisfy the filing obligation of the registrant under any of the following provisions:

On January 24, 2012, ACI Worldwide, Inc. (“ACI”) announced that (i) ACI and S1 Corporation (the “Company”) had entered into an amendment to that certain timing agreement previously entered into with the Antitrust Division of the United States Department of Justice (the “Antitrust Division”) in order to extend from January 31, 2012 to February 10, 2012 the date until which ACI and the Company will not consummate the pending transaction without the consent of the Antitrust Division, and (ii) ACI had extended its exchange offer for all of the outstanding shares of the Company to 5:00 p.m., Eastern Time, on Friday, February 10, 2012, unless further extended. There can be no assurance that ACI and the Company will obtain the regulatory approvals necessary to consummate the transaction or as to the timing of any such approvals. A copy of ACI’s press release is attached as Exhibit 99.1 hereto and is incorporated by reference herein.

Forward Looking Statements

Certain statements herein may constitute “forward-looking statements” within the meaning of the Private Securities Litigation Reform Act of 1995. Such statements include, but are not limited to, those regarding any transaction with ACI and other statements that are not historical facts. These statements involve risks and uncertainties including those detailed in S1’s Annual Reports on Form 10-K and other filings with the Securities and Exchange Commission. Should one or more of these risks or uncertainties materialize, or should underlying assumptions prove incorrect, actual outcomes may vary materially from those forecasted or expected. S1 disclaims any intention or obligation to update publicly or revise such statements, whether as a result of new information, future events or otherwise.

Additional Information and Where to Find It

S1 Corporation has filed a Solicitation/Recommendation Statement on Schedule 14D-9, as amended, with the SEC. INVESTORS AND STOCKHOLDERS ARE URGED TO READ THAT STATEMENT AND OTHER MATERIALS FILED WITH THE SEC BY S1 BECAUSE THEY CONTAIN IMPORTANT INFORMATION. S1 stockholders and other interested parties may obtain, free of charge, copies of S1’s Schedule 14D-9, as amended, and other documents filed by S1 with the SEC at the SEC’s website at http://www.sec.gov. In addition free copies of the documents filed by S1 with the SEC with respect to the exchange offer may be obtained by contacting S1’s Investor Relations at (404) 923-3500 or by accessing S1’s investor relations website at www.s1.com.

SIGNATURES

Pursuant to the requirements of the Securities Exchange Act of 1934, the registrant has duly caused this report to be signed on its behalf by the undersigned hereunto duly authorized.

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