<PAGE>   1
    As filed with the Securities and Exchange Commission on August 9, 2000
                                          Registration Statement No. 333-82383
================================================================================

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

                        -----------------------------

                        POST-EFFECTIVE AMENDMENT NO. 1 TO
                                    FORM S-8

                             REGISTRATION STATEMENT
                        UNDER THE SECURITIES ACT OF 1933

                        -----------------------------

                                S1 CORPORATION
            (Exact name of registrant as specified in its charter)

                    Delaware                               58-2395199
                 ------------                            -------------
(State or other jurisdiction of incorporation or  (IRS Employer Identification
                 organization)                                No.)

         3390 Peachtree Road, NE, Suite 1700, Atlanta, Georgia 30326
                   (Address of Principal Executive Offices)

                        -----------------------------

   SECOND AMENDED AND RESTATED S1 CORPORATION EMPLOYEE STOCK PURCHASE PLAN
                           (Full title of the plan)

                          -------------------------

                               Robert F. Stockwell
                             Chief Financial Officer
                                 S1 Corporation
                       3390 Peachtree Road, NE, Suite 1700
                            Atlanta, Georgia 30326
                   (Name and address of agent for service)

                                 (404) 812-6780
        (Telephone number, including area code, of agent for service)

                                    COPY TO:

                             Stuart G. Stein, Esq.
                             Hogan & Hartson L.L.P.
                           555 Thirteenth Street, N.W.
                             Washington, D.C. 20004
                                 (202) 637-8575

                          --------------------------

                         CALCULATION OF REGISTRATION FEE

<TABLE>
<CAPTION>
=========================================================================================
                                             PROPOSED       PROPOSED          AMOUNT OF
                                AMOUNT       MAXIMUM        MAXIMUM         REGISTRATION
     TITLE OF SECURITIES         TO BE    OFFERING PRICE   AGGREGATE            FEE
      TO BE REGISTERED        REGISTERED    PER SHARE       OFFERING           PRICE
-----------------------------------------------------------------------------------------
<S>                           <C>            <C>          <C>                 <C>
Common Stock, par value       10,892,490     $15.78125*   $171,897,107.81*    $45,380.84*
$0.01 per share
-----------------------------------------------------------------------------------------
=========================================================================================
</TABLE>

*  Estimated solely for the purpose of calculating this fee pursuant to Rule
   457(c) and (h) under the Securities Act of 1933, as amended, based upon the
   average of the high and low prices for shares of common stock S1 Corporation
   as reported on The Nasdaq Stock Market's National Market Tier of $15.78125 on
   the basis of August 2, 2000. In addition, pursuant to Rule 416(c) under the
   Securities Act of 1933, this registration statement also covers an
   indeterminate amount of interests to be offered or sold pursuant to the
   employee benefit plan described herein.


================================================================================


<PAGE>   2

                         POST-EFFECTIVE AMENDMENT NO. 1
                                       TO
                                    Form S-8
                             REGISTRATION STATEMENT


                                     PART I

             INFORMATION REQUIRED IN THE SECTION 10(a) PROSPECTUS

      The documents containing the information specified in Part I will be sent
or given to employees as specified by Rule 428(b)(1) under the Securities Act of
1933. In accordance with the instructions to Part I of Form S-8 as promulgated
by the Securities and Exchange Commission, these documents will not be filed
with the SEC either as part of this registration statement or as prospectuses or
prospectus supplements pursuant to Rule 424 under the Securities Act. These
documents and the documents incorporated by reference into this registration
statement pursuant to Item 3 of Part II of this registration statement, taken
together, constitute the prospectus that meets the requirements of Section 10(a)
of the Securities Act.


                                     PART II

              INFORMATION REQUIRED IN THE REGISTRATION STATEMENT

ITEM 3.     INCORPORATION OF DOCUMENTS BY REFERENCE.

      S1 Corporation, which we refer to as S1 or the registrant, and the Second
Amended and Restated S1 Corporation Employee Stock Purchase Plan, which we refer
to as the Plan, hereby incorporate by reference into this registration statement
the following documents filed by them with the SEC:

            (a)   The Annual Report on Form 10-K of S1 for the fiscal year ended
                  December 31, 1999.

            (b)   S1's Quarterly Report on Form 10-Q for the quarterly period
                  ended March 31, 2000.

            (c)   Current Report on Form 8-K filed on March 8, 2000
                  Current Report on Form 8-K filed on May 2, 2000
                  Current Report on Form 8-K filed on May 8, 2000
                  Current Report on Form 8-K filed on May 26, 2000
                  Current Report on Form 8-K filed on June 7, 2000
                  Current Report on Form 8-K filed on August 9, 2000

            (d)   The description of S1's common stock, par value $0.01 per
                  share contained in S1's Registration Statement on Form 8-A
                  filed with the SEC on September 30, 1998, including any
                  amendment or report filed for the purpose of updating such
                  description.

      In addition, all documents filed by S1 or the Plan subsequent to the date
hereof pursuant to Sections 13(a), 13(c), 14 and 15(d) of the Exchange Act prior
to the filing of a post-effective amendment which indicates that all securities
offered have been sold or which deregisters all securities then remaining
unsold, shall be deemed to be incorporated by reference in this registration
statement and to be part of this registration statement from the date of filing
of those documents. Any statement contained in a document incorporated or deemed
to be incorporated by reference into


                                       1

<PAGE>   3
this registration statement shall be deemed to be modified or superseded for
purposes of this registration statement to the extent that a statement
contained in this registration statement or in any other subsequently filed
document which also is or is deemed to be incorporated by reference into this
registration statement modifies or supersedes such statement. Any such
statement so modified or superseded shall not be deemed, except as so modified
or superseded, to constitute a part of this registration statement.


ITEM 4.     DESCRIPTION OF SECURITIES.

      Not applicable.


ITEM 5.     INTERESTS OF NAMED EXPERTS AND COUNSEL.

      Not applicable.


ITEM 6.     INDEMNIFICATION OF DIRECTORS AND OFFICERS.

      Section 145 of the General Corporation Law of the State of Delaware sets
forth certain circumstances under which directors, officers, employees and
agents may be indemnified against liability that they may incur in their
capacity as such. Section 6 of S1's Amended and Restated Certificate of
Incorporation, as amended, provides for indemnification of S1's directors,
officers, employees and agents under certain circumstances. Section 6 of S1's
Amended and Restated Bylaws, as amended, provides that S1 may purchase and
maintain insurance on behalf of directors, officers, employees or agents. S1 has
in effect a policy of liability insurance covering its directors and officers.
Insofar as indemnification for liabilities arising under the Securities Act may
be permitted to directors, officers and controlling persons of S1 pursuant to
the foregoing provisions, or otherwise, S1 has been advised that in the opinion
of the SEC such indemnification is against public policy as expressed in the
Securities Act and is, therefore, unenforceable. In the event that a claim for
indemnification against such liabilities (other than the payment by S1 of
expenses incurred or paid by a director, officer, or controlling person of S1 in
the successful defense of any action, suit, or proceeding) is asserted by such
director, officer, or controlling person in connection with the securities being
registered, S1 will, unless in the opinion of its counsel the matter has been
settled by controlling precedent, submit to a court of appropriate jurisdiction
the question whether such indemnification by it is against public policy as
expressed in the Securities Act and will be governed by the final adjudication
of such issue.


ITEM 7.     EXEMPTION FROM REGISTRATION CLAIMED.

      Not applicable.


ITEM 8.     EXHIBITS.

Exhibit
No.         Description

4.1         Amended and Restated Certificate of Incorporation of S1 Corporation
            ("S1") (filed as Exhibit 1 to S1's Registration Statement on Form
            8-A filed with the Securities and Exchange Commission (the "SEC") on
            September 30, 1998 and incorporated herein by reference).



                                       2
<PAGE>   4

4.2         Certificate of Amendment of Amended and Restated Certificate of
            Incorporation of S1 dated June 3, 1999 (filed as Exhibit 4.2 to S1's
            Registration Statement on Form S-8 (File No. 333-82369) filed with
            the SEC on July 7, 1999 and incorporated herein by reference).

4.3         Certificate of Amendment of Amended and Restated Certificate of
            Incorporation of S1 dated November 10, 1999 (filed as Exhibit 3.3 to
            S1's Annual Report on Form 10-K for the fiscal year ending December
            31, 1999 and incorporated herein by reference).

4.4         Certificate of Designation for S1's Series B Redeemable Convertible
            Preferred Stock (filed as Exhibit 2 to S1's Registration Statement
            on Form 8-A filed with the SEC on September 30, 1998 and
            incorporated herein by reference).

4.5         Certificate of Designation for S1's Series C Redeemable Convertible
            Preferred Stock (filed as Exhibit 3 to S1's Quarterly Report on Form
            10-Q for the quarterly period ending March 31, 1999 and incorporated
            herein by reference).

4.6         Certificate of Designations for S1's Series D Convertible Preferred
            Stock (filed as Exhibit 3 to S1's Current Report on Form 8-K filed
            with the SEC on June 7, 2000 and incorporated herein by reference).

4.7         Amended and Restated Bylaws of S1, as amended.

4.8         Specimen certificate for S1's common stock (filed as Exhibit 4 to
            S1's Quarterly Report on Form 10-Q for the quarterly period ending
            March 31, 2000 and incorporated herein by reference).

4.9         Specimen certificate for S1's Series B Convertible Redeemable
            Preferred Stock (filed as Exhibit 4.3 to S1's Annual Report on Form
            10-K for the fiscal year ended December 31, 1998 and incorporated
            herein by reference).

4.10        Specimen certificate for S1's Series C Redeemable Convertible
            Preferred Stock.

4.11        Specimen certificate for S1's Series D Convertible Preferred Stock
            (filed as Exhibit 4 to S1's Current Report on Form 8-K filed with
            the SEC on June 7, 2000 and incorporated herein by reference).

5           Opinion of Nancy K. Kenley as to the legality of the securities
            registered hereunder, including the consent of Ms. Kenley.

23.1        Consent of Nancy K. Kenley (included in Item 5).

23.2        Consent of PricewaterhouseCoopers LLP.

23.3        Consent of KPMG LLP.

24          Power of Attorney.

ITEM 9.     UNDERTAKINGS.

            (a)   The undersigned registrant hereby undertakes:

                  (1) To file, during any period in which offers or sales are
                  being made, a post-effective amendment to this registration
                  statement:



                                       3
<PAGE>   5
                        (i)   To include any prospectus required by section
                              10(a)(3) of the Securities Act;

                        (ii)  To reflect in the prospectus any facts or events
                              arising after the effective date of the
                              registration statement (or the most recent
                              post-effective amendment thereof) which,
                              individually or in the aggregate, represent a
                              fundamental change in the information set forth in
                              the registration statement; and

                        (iii) To include any material information with respect
                              to the plan of distribution not previously
                              disclosed in the registration statement or any
                              material change to such information in the
                              registration statement.

                        provided, however, that paragraphs (a)(1)(i) and
                  (a)(1)(ii) do not apply if the information required to be
                  included in a post-effective amendment by those paragraphs is
                  contained in periodic reports filed by the registrant pursuant
                  to section 13 or section 15(d) of the Exchange Act that are
                  incorporated by reference in the registration statement.

                        (2) That, for the purpose of determining any liability
                  under the Securities Act, each such post-effective amendment
                  shall be deemed to be a new registration statement relating to
                  the securities offered therein, and the offering of such
                  securities at that time shall be deemed to be the initial bona
                  fide offering thereof.

                        (3) To remove from registration by means of a
                  post-effective amendment any of the securities being
                  registered which remain unsold at the termination of the
                  offering.

            (b) The undersigned registrant hereby undertakes that, for purposes
      of determining any liability under the Securities Act, each filing of the
      registrant's annual report pursuant to section 13(a) or section 15(d) of
      the Exchange Act (and, where applicable, each filing of an employee
      benefit plan's annual report pursuant to section 15(d) of the Exchange
      Act) that is incorporated by reference in the registration statement shall
      be deemed to be a new registration statement relating to the securities
      offered therein, and the offering of such securities at that time shall be
      deemed to be the initial bona fide offering thereof.

            (c)   The undertaking concerning indemnification is set forth
      under the response to Item 6.



                                       4
<PAGE>   6

                                   SIGNATURES

      The Registrant. Pursuant to the requirements of the Securities Act of
1933, the registrant certifies that it has reasonable grounds to believe that it
meets all of the requirements for filing on Form S-8 and has duly caused this
registration statement to be signed on its behalf by the undersigned, thereunto
duly authorized, in the City of Atlanta, State of Georgia, on this 9th day of
August, 2000.

                                          S1 CORPORATION



                                          By:  /s/ Robert F. Stockwell
                                               ---------------------------
                                               Robert F. Stockwell
                                               Chief Financial Officer


      Pursuant to the requirements of the Securities Act of 1933, this
registration statement has been signed by the following persons in the
capacities indicated below on this 9th day of August, 2000.

      SIGNATURE                           TITLE

/s/ James S. Mahan, III             Chief Executive Officer
------------------------------      and a Director
James S. Mahan, III                 (Principal Executive Officer)



/s/ Robert F. Stockwell             Chief Financial Officer
------------------------------      (Principal Financial Officer
Robert F. Stockwell                 and Principal Accounting Officer)



/s/ Michel Akkermans*               Chairman of the Board
------------------------------
Michel Akkermans


/s/ Robert W. Copelan*              Director
------------------------------
Robert W. Copelan


/s/ Gregg S. Freishtat*             Director
------------------------------
Gregg S. Freishtat


/s/ David C. Hodgson*               Director
------------------------------
David C. Hodgson


/s/ Joseph S. McCall*               Director
------------------------------
Joseph S. McCall


/s/ Howard J. Runnion*              Director
------------------------------
Howard J. Runnion, Jr.

<PAGE>   7

/s/ Jackson L. Wilson*              Director
------------------------------
Jackson L. Wilson, Jr.



*By /s/ Robert F. Stockwell
   --------------------------
   Robert F. Stockwell
   pursuant to Power of Attorney



<PAGE>   8


      The Plan. Pursuant to the requirements of the Securities Act of 1933, the
trustees (or other persons who administer the employee benefit plan) have duly
caused this registration statement to be signed on its behalf by the
undersigned, thereunto duly authorized, in the City of Atlanta, State of
Georgia, on this 9th day of August, 2000.

                        SECOND AMENDED AND RESTATED S1 CORPORATION
                        EMPLOYEE STOCK PURCHASE PLAN



                                          By:  /s/ Robert F. Stockwell
                                               -------------------------
                                               Robert F. Stockwell
                                               on behalf of the
                                               Second Amended and Restated S1
                                               Corporation Employee Stock
                                               Purchase Plan


<PAGE>   9



                                  EXHIBIT INDEX

Exhibit
No.                                 Description

4.1         Amended and Restated Certificate of Incorporation of S1 Corporation
            ("S1") (filed as Exhibit 1 to S1's Registration Statement on Form
            8-A filed with the Securities and Exchange Commission (the "SEC") on
            September 30, 1998 and incorporated herein by reference).

4.2         Certificate of Amendment of Amended and Restated Certificate of
            Incorporation of S1 dated June 3, 1999 (filed as Exhibit 4.2 to S1's
            Registration Statement on Form S-8 (File No. 333-82369) filed with
            the SEC on July 7, 1999 and incorporated herein by reference).

4.3         Certificate of Amendment of Amended and Restated Certificate of
            Incorporation of S1 dated November 10, 1999 (filed as Exhibit 3.3 to
            S1's Annual Report on Form 10-K for the fiscal year ending December
            31, 1999 and incorporated herein by reference).

4.4         Certificate of Designation for S1's Series B Redeemable Convertible
            Preferred Stock (filed as Exhibit 2 to S1's Registration Statement
            on Form 8-A filed with the SEC on September 30, 1998 and
            incorporated herein by reference).

4.5         Certificate of Designation for S1's Series C Redeemable Convertible
            Preferred Stock (filed as Exhibit 3 to S1's Quarterly Report on Form
            10-Q for the quarterly period ending March 31, 1999 and incorporated
            herein by reference).

4.6         Certificate of Designations for S1's Series D Convertible Preferred
            Stock (filed as Exhibit 3 to S1's Current Report on Form 8-K filed
            with the SEC on June 7, 2000 and incorporated herein by reference).

4.7         Amended and Restated Bylaws of S1, as amended.

4.8         Specimen certificate for S1's common stock (filed as Exhibit 4 to
            S1's Quarterly Report on Form 10-Q for the quarterly period ending
            March 31, 2000 and incorporated herein by reference).

4.9         Specimen certificate for S1's Series B Convertible Redeemable
            Preferred Stock (filed as Exhibit 4.3 to S1's Annual Report on Form
            10-K for the fiscal year ended December 31, 1998 and incorporated
            herein by reference).

4.10        Specimen certificate for S1's Series C Redeemable Convertible
            Preferred Stock.

4.11        Specimen certificate for S1's Series D Convertible Preferred Stock
            (filed as Exhibit 4 to S1's Current Report on Form 8-K filed with
            the SEC on June 7, 2000 and incorporated herein by reference).

5           Opinion of Nancy K. Kenley as to the legality of the securities
            registered hereunder, including the consent of Ms. Kenley.

23.1        Consent of Nancy K. Kenley (included in Item 5).

23.2        Consent of PricewaterhouseCoopers LLP.

<PAGE>   10
23.3        Consent of KPMG LLP.

24          Power of Attorney.
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{"company-data": {"conformed-name": "S1 CORP /DE/", "cik": "0001063254", "assigned-sic": "7370", "irs-number": "582395199", "state-of-incorporation": "DE", "fiscal-year-end": "1231"}, "filing-values": {"form-type": "S-8", "act": "33", "file-number": "333-43370", "film-number": "690065"}, "business-address": {"street1": "3390 PEACHTREE ROAD NE", "street2": "SUITE 1700", "city": "ATLANTA", "state": "GA", "zip": "30326", "phone": "4048126200"}, "mail-address": {"street1": "3390 PEACHTREE ROAD NE", "street2": "SUITE 1700", "city": "ATLANTA", "state": "GA", "zip": "30326"}, "former-company": {"former-conformed-name": "SECURITY FIRST TECHNOLOGIES CORP", "date-changed": "19980603"}}
1
    As filed with the Securities and Exchange Commission on August 9, 2000
                                          Registration Statement No. 333-82383
================================================================================

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

                        -----------------------------

                        POST-EFFECTIVE AMENDMENT NO. 1 TO
                                    FORM S-8

                             REGISTRATION STATEMENT
                        UNDER THE SECURITIES ACT OF 1933

                        -----------------------------

                                S1 CORPORATION
            (Exact name of registrant as specified in its charter)

                    Delaware                               58-2395199
                 ------------                            -------------
(State or other jurisdiction of incorporation or  (IRS Employer Identification
                 organization)                                No.)

         3390 Peachtree Road, NE, Suite 1700, Atlanta, Georgia 30326
                   (Address of Principal Executive Offices)

                        -----------------------------

   SECOND AMENDED AND RESTATED S1 CORPORATION EMPLOYEE STOCK PURCHASE PLAN
                           (Full title of the plan)

                          -------------------------

                               Robert F. Stockwell
                             Chief Financial Officer
                                 S1 Corporation
                       3390 Peachtree Road, NE, Suite 1700
                            Atlanta, Georgia 30326
                   (Name and address of agent for service)

                                 (404) 812-6780
        (Telephone number, including area code, of agent for service)

                                    COPY TO:

                             Stuart G. Stein, Esq.
                             Hogan & Hartson L.L.P.
                           555 Thirteenth Street, N.W.
                             Washington, D.C. 20004
                                 (202) 637-8575

                          --------------------------

                         CALCULATION OF REGISTRATION FEE
=========================================================================================
                                             PROPOSED       PROPOSED          AMOUNT OF
                                AMOUNT       MAXIMUM        MAXIMUM         REGISTRATION
     TITLE OF SECURITIES         TO BE    OFFERING PRICE   AGGREGATE            FEE
      TO BE REGISTERED        REGISTERED    PER SHARE       OFFERING           PRICE
-----------------------------------------------------------------------------------------
Common Stock, par value       10,892,490     $15.78125*   $171,897,107.81*    $45,380.84*
$0.01 per share
-----------------------------------------------------------------------------------------
=========================================================================================
*  Estimated solely for the purpose of calculating this fee pursuant to Rule
   457(c) and (h) under the Securities Act of 1933, as amended, based upon the
   average of the high and low prices for shares of common stock S1 Corporation
   as reported on The Nasdaq Stock Market's National Market Tier of $15.78125 on
   the basis of August 2, 2000. In addition, pursuant to Rule 416(c) under the
   Securities Act of 1933, this registration statement also covers an
   indeterminate amount of interests to be offered or sold pursuant to the
   employee benefit plan described herein.

================================================================================
2

                         POST-EFFECTIVE AMENDMENT NO. 1
                                       TO
                                    Form S-8
                             REGISTRATION STATEMENT

                                     PART I

             INFORMATION REQUIRED IN THE SECTION 10(a) PROSPECTUS

      The documents containing the information specified in Part I will be sent
or given to employees as specified by Rule 428(b)(1) under the Securities Act of
1933. In accordance with the instructions to Part I of Form S-8 as promulgated
by the Securities and Exchange Commission, these documents will not be filed
with the SEC either as part of this registration statement or as prospectuses or
prospectus supplements pursuant to Rule 424 under the Securities Act. These
documents and the documents incorporated by reference into this registration
statement pursuant to Item 3 of Part II of this registration statement, taken
together, constitute the prospectus that meets the requirements of Section 10(a)
of the Securities Act.

                                     PART II

              INFORMATION REQUIRED IN THE REGISTRATION STATEMENT

ITEM 3.     INCORPORATION OF DOCUMENTS BY REFERENCE.

      S1 Corporation, which we refer to as S1 or the registrant, and the Second
Amended and Restated S1 Corporation Employee Stock Purchase Plan, which we refer
to as the Plan, hereby incorporate by reference into this registration statement
the following documents filed by them with the SEC:

            (a)   The Annual Report on Form 10-K of S1 for the fiscal year ended
                  December 31, 1999.

            (b)   S1's Quarterly Report on Form 10-Q for the quarterly period
                  ended March 31, 2000.

            (c)   Current Report on Form 8-K filed on March 8, 2000
                  Current Report on Form 8-K filed on May 2, 2000
                  Current Report on Form 8-K filed on May 8, 2000
                  Current Report on Form 8-K filed on May 26, 2000
                  Current Report on Form 8-K filed on June 7, 2000
                  Current Report on Form 8-K filed on August 9, 2000

            (d)   The description of S1's common stock, par value $0.01 per
                  share contained in S1's Registration Statement on Form 8-A
                  filed with the SEC on September 30, 1998, including any
                  amendment or report filed for the purpose of updating such
                  description.

      In addition, all documents filed by S1 or the Plan subsequent to the date
hereof pursuant to Sections 13(a), 13(c), 14 and 15(d) of the Exchange Act prior
to the filing of a post-effective amendment which indicates that all securities
offered have been sold or which deregisters all securities then remaining
unsold, shall be deemed to be incorporated by reference in this registration
statement and to be part of this registration statement from the date of filing
of those documents. Any statement contained in a document incorporated or deemed
to be incorporated by reference into

                                       1
3
this registration statement shall be deemed to be modified or superseded for
purposes of this registration statement to the extent that a statement
contained in this registration statement or in any other subsequently filed
document which also is or is deemed to be incorporated by reference into this
registration statement modifies or supersedes such statement. Any such
statement so modified or superseded shall not be deemed, except as so modified
or superseded, to constitute a part of this registration statement.

ITEM 4.     DESCRIPTION OF SECURITIES.

      Not applicable.

ITEM 5.     INTERESTS OF NAMED EXPERTS AND COUNSEL.

      Not applicable.

ITEM 6.     INDEMNIFICATION OF DIRECTORS AND OFFICERS.

      Section 145 of the General Corporation Law of the State of Delaware sets
forth certain circumstances under which directors, officers, employees and
agents may be indemnified against liability that they may incur in their
capacity as such. Section 6 of S1's Amended and Restated Certificate of
Incorporation, as amended, provides for indemnification of S1's directors,
officers, employees and agents under certain circumstances. Section 6 of S1's
Amended and Restated Bylaws, as amended, provides that S1 may purchase and
maintain insurance on behalf of directors, officers, employees or agents. S1 has
in effect a policy of liability insurance covering its directors and officers.
Insofar as indemnification for liabilities arising under the Securities Act may
be permitted to directors, officers and controlling persons of S1 pursuant to
the foregoing provisions, or otherwise, S1 has been advised that in the opinion
of the SEC such indemnification is against public policy as expressed in the
Securities Act and is, therefore, unenforceable. In the event that a claim for
indemnification against such liabilities (other than the payment by S1 of
expenses incurred or paid by a director, officer, or controlling person of S1 in
the successful defense of any action, suit, or proceeding) is asserted by such
director, officer, or controlling person in connection with the securities being
registered, S1 will, unless in the opinion of its counsel the matter has been
settled by controlling precedent, submit to a court of appropriate jurisdiction
the question whether such indemnification by it is against public policy as
expressed in the Securities Act and will be governed by the final adjudication
of such issue.

ITEM 7.     EXEMPTION FROM REGISTRATION CLAIMED.

      Not applicable.

ITEM 8.     EXHIBITS.

Exhibit
No.         Description

4.1         Amended and Restated Certificate of Incorporation of S1 Corporation
            ("S1") (filed as Exhibit 1 to S1's Registration Statement on Form
            8-A filed with the Securities and Exchange Commission (the "SEC") on
            September 30, 1998 and incorporated herein by reference).

                                       2
4

4.2         Certificate of Amendment of Amended and Restated Certificate of
            Incorporation of S1 dated June 3, 1999 (filed as Exhibit 4.2 to S1's
            Registration Statement on Form S-8 (File No. 333-82369) filed with
            the SEC on July 7, 1999 and incorporated herein by reference).

4.3         Certificate of Amendment of Amended and Restated Certificate of
            Incorporation of S1 dated November 10, 1999 (filed as Exhibit 3.3 to
            S1's Annual Report on Form 10-K for the fiscal year ending December
            31, 1999 and incorporated herein by reference).

4.4         Certificate of Designation for S1's Series B Redeemable Convertible
            Preferred Stock (filed as Exhibit 2 to S1's Registration Statement
            on Form 8-A filed with the SEC on September 30, 1998 and
            incorporated herein by reference).

4.5         Certificate of Designation for S1's Series C Redeemable Convertible
            Preferred Stock (filed as Exhibit 3 to S1's Quarterly Report on Form
            10-Q for the quarterly period ending March 31, 1999 and incorporated
            herein by reference).

4.6         Certificate of Designations for S1's Series D Convertible Preferred
            Stock (filed as Exhibit 3 to S1's Current Report on Form 8-K filed
            with the SEC on June 7, 2000 and incorporated herein by reference).

4.7         Amended and Restated Bylaws of S1, as amended.

4.8         Specimen certificate for S1's common stock (filed as Exhibit 4 to
            S1's Quarterly Report on Form 10-Q for the quarterly period ending
            March 31, 2000 and incorporated herein by reference).

4.9         Specimen certificate for S1's Series B Convertible Redeemable
            Preferred Stock (filed as Exhibit 4.3 to S1's Annual Report on Form
            10-K for the fiscal year ended December 31, 1998 and incorporated
            herein by reference).

4.10        Specimen certificate for S1's Series C Redeemable Convertible
            Preferred Stock.

4.11        Specimen certificate for S1's Series D Convertible Preferred Stock
            (filed as Exhibit 4 to S1's Current Report on Form 8-K filed with
            the SEC on June 7, 2000 and incorporated herein by reference).

5           Opinion of Nancy K. Kenley as to the legality of the securities
            registered hereunder, including the consent of Ms. Kenley.

23.1        Consent of Nancy K. Kenley (included in Item 5).

23.2        Consent of PricewaterhouseCoopers LLP.

23.3        Consent of KPMG LLP.

24          Power of Attorney.

ITEM 9.     UNDERTAKINGS.

            (a)   The undersigned registrant hereby undertakes:

                  (1) To file, during any period in which offers or sales are
                  being made, a post-effective amendment to this registration
                  statement:

                                       3
5
                        (i)   To include any prospectus required by section
                              10(a)(3) of the Securities Act;

                        (ii)  To reflect in the prospectus any facts or events
                              arising after the effective date of the
                              registration statement (or the most recent
                              post-effective amendment thereof) which,
                              individually or in the aggregate, represent a
                              fundamental change in the information set forth in
                              the registration statement; and

                        (iii) To include any material information with respect
                              to the plan of distribution not previously
                              disclosed in the registration statement or any
                              material change to such information in the
                              registration statement.

                        provided, however, that paragraphs (a)(1)(i) and
                  (a)(1)(ii) do not apply if the information required to be
                  included in a post-effective amendment by those paragraphs is
                  contained in periodic reports filed by the registrant pursuant
                  to section 13 or section 15(d) of the Exchange Act that are
                  incorporated by reference in the registration statement.

                        (2) That, for the purpose of determining any liability
                  under the Securities Act, each such post-effective amendment
                  shall be deemed to be a new registration statement relating to
                  the securities offered therein, and the offering of such
                  securities at that time shall be deemed to be the initial bona
                  fide offering thereof.

                        (3) To remove from registration by means of a
                  post-effective amendment any of the securities being
                  registered which remain unsold at the termination of the
                  offering.

            (b) The undersigned registrant hereby undertakes that, for purposes
      of determining any liability under the Securities Act, each filing of the
      registrant's annual report pursuant to section 13(a) or section 15(d) of
      the Exchange Act (and, where applicable, each filing of an employee
      benefit plan's annual report pursuant to section 15(d) of the Exchange
      Act) that is incorporated by reference in the registration statement shall
      be deemed to be a new registration statement relating to the securities
      offered therein, and the offering of such securities at that time shall be
      deemed to be the initial bona fide offering thereof.

            (c)   The undertaking concerning indemnification is set forth
      under the response to Item 6.

                                       4
6

                                   SIGNATURES

      The Registrant. Pursuant to the requirements of the Securities Act of
1933, the registrant certifies that it has reasonable grounds to believe that it
meets all of the requirements for filing on Form S-8 and has duly caused this
registration statement to be signed on its behalf by the undersigned, thereunto
duly authorized, in the City of Atlanta, State of Georgia, on this 9th day of
August, 2000.

                                          S1 CORPORATION

                                          By:  /s/ Robert F. Stockwell
                                               ---------------------------
                                               Robert F. Stockwell
                                               Chief Financial Officer

      Pursuant to the requirements of the Securities Act of 1933, this
registration statement has been signed by the following persons in the
capacities indicated below on this 9th day of August, 2000.

      SIGNATURE                           TITLE

/s/ James S. Mahan, III             Chief Executive Officer
------------------------------      and a Director
James S. Mahan, III                 (Principal Executive Officer)

/s/ Robert F. Stockwell             Chief Financial Officer
------------------------------      (Principal Financial Officer
Robert F. Stockwell                 and Principal Accounting Officer)

/s/ Michel Akkermans*               Chairman of the Board
------------------------------
Michel Akkermans

/s/ Robert W. Copelan*              Director
------------------------------
Robert W. Copelan

/s/ Gregg S. Freishtat*             Director
------------------------------
Gregg S. Freishtat

/s/ David C. Hodgson*               Director
------------------------------
David C. Hodgson

/s/ Joseph S. McCall*               Director
------------------------------
Joseph S. McCall

/s/ Howard J. Runnion*              Director
------------------------------
Howard J. Runnion, Jr.
7

/s/ Jackson L. Wilson*              Director
------------------------------
Jackson L. Wilson, Jr.

*By /s/ Robert F. Stockwell
   --------------------------
   Robert F. Stockwell
   pursuant to Power of Attorney
8

      The Plan. Pursuant to the requirements of the Securities Act of 1933, the
trustees (or other persons who administer the employee benefit plan) have duly
caused this registration statement to be signed on its behalf by the
undersigned, thereunto duly authorized, in the City of Atlanta, State of
Georgia, on this 9th day of August, 2000.

                        SECOND AMENDED AND RESTATED S1 CORPORATION
                        EMPLOYEE STOCK PURCHASE PLAN

                                          By:  /s/ Robert F. Stockwell
                                               -------------------------
                                               Robert F. Stockwell
                                               on behalf of the
                                               Second Amended and Restated S1
                                               Corporation Employee Stock
                                               Purchase Plan
9

                                  EXHIBIT INDEX

Exhibit
No.                                 Description

4.1         Amended and Restated Certificate of Incorporation of S1 Corporation
            ("S1") (filed as Exhibit 1 to S1's Registration Statement on Form
            8-A filed with the Securities and Exchange Commission (the "SEC") on
            September 30, 1998 and incorporated herein by reference).

4.2         Certificate of Amendment of Amended and Restated Certificate of
            Incorporation of S1 dated June 3, 1999 (filed as Exhibit 4.2 to S1's
            Registration Statement on Form S-8 (File No. 333-82369) filed with
            the SEC on July 7, 1999 and incorporated herein by reference).

4.3         Certificate of Amendment of Amended and Restated Certificate of
            Incorporation of S1 dated November 10, 1999 (filed as Exhibit 3.3 to
            S1's Annual Report on Form 10-K for the fiscal year ending December
            31, 1999 and incorporated herein by reference).

4.4         Certificate of Designation for S1's Series B Redeemable Convertible
            Preferred Stock (filed as Exhibit 2 to S1's Registration Statement
            on Form 8-A filed with the SEC on September 30, 1998 and
            incorporated herein by reference).

4.5         Certificate of Designation for S1's Series C Redeemable Convertible
            Preferred Stock (filed as Exhibit 3 to S1's Quarterly Report on Form
            10-Q for the quarterly period ending March 31, 1999 and incorporated
            herein by reference).

4.6         Certificate of Designations for S1's Series D Convertible Preferred
            Stock (filed as Exhibit 3 to S1's Current Report on Form 8-K filed
            with the SEC on June 7, 2000 and incorporated herein by reference).

4.7         Amended and Restated Bylaws of S1, as amended.

4.8         Specimen certificate for S1's common stock (filed as Exhibit 4 to
            S1's Quarterly Report on Form 10-Q for the quarterly period ending
            March 31, 2000 and incorporated herein by reference).

4.9         Specimen certificate for S1's Series B Convertible Redeemable
            Preferred Stock (filed as Exhibit 4.3 to S1's Annual Report on Form
            10-K for the fiscal year ended December 31, 1998 and incorporated
            herein by reference).

4.10        Specimen certificate for S1's Series C Redeemable Convertible
            Preferred Stock.

4.11        Specimen certificate for S1's Series D Convertible Preferred Stock
            (filed as Exhibit 4 to S1's Current Report on Form 8-K filed with
            the SEC on June 7, 2000 and incorporated herein by reference).

5           Opinion of Nancy K. Kenley as to the legality of the securities
            registered hereunder, including the consent of Ms. Kenley.

23.1        Consent of Nancy K. Kenley (included in Item 5).

23.2        Consent of PricewaterhouseCoopers LLP.
10
23.3        Consent of KPMG LLP.

24          Power of Attorney.