<PAGE>   1
     AS FILED WITH THE SECURITIES AND EXCHANGE COMMISSION ON MAY 15, 2000
                                                    Registration No. 333-_______
================================================================================

                       SECURITIES AND EXCHANGE COMMISSION
                             WASHINGTON, D.C. 20549
                            ------------------------

                                    FORM S-8
                          REGISTRATION STATEMENT UNDER
                           THE SECURITIES ACT OF 1933

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

                                    DELAWARE
                         (State or other jurisdiction of
                         incorporation or organization)

                                   58-2395199
                      (IRS employer identification number)

                             3390 Peachtree Road, NE
                                   Suite 1700
                             Atlanta, Georgia 30326
                                 (404) 812-6200
               (Address, including zip code, and telephone number,
                 including area code, of registrant's principal
                               executive offices)

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

                    Q-Up Systems, Inc. 1999 Stock Option Plan
                            (Full title of the Plan)

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

                               Robert F. Stockwell
                             Chief Financial Officer
                                 S1 Corporation
                             3390 Peachtree Road, NE
                                   Suite 1700
                             Atlanta, Georgia 30326
                                 (404) 812-6780
            (Name, address and telephone number of Agent for Service)

                                    Copy to:
                              Stuart G. Stein, Esq.
                             Hogan & Hartson L.L.P.
                           555 Thirteenth Street, N.W.
                           Washington, D.C. 20004-1109
                                 (202) 637-8575

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



                         CALCULATION OF REGISTRATION FEE

<TABLE>
<CAPTION>
===========================================================================================================================
                                                AMOUNT         PROPOSED MAXIMUM         PROPOSED MAXIMUM       AMOUNT OF
                  TITLE OF SECURITIES            TO BE          OFFERING PRICE         AGGREGATE OFFERING     REGISTRATION
                    TO BE REGISTERED         REGISTERED(2)      PER SHARE (1)             PRICE (1)(2)         FEE (1)(2)
---------------------------------------------------------------------------------------------------------------------------
<S>                                         <C>                <C>                     <C>                    <C>
Common Stock                                 1,442,116          35.78125                51,600,713.13          13622.59
===========================================================================================================================
</TABLE>

(1) Estimated solely for the purpose of calculating the registration fee
pursuant to Rule 457(h) of the Securities Act of 1933, as amended.

(2) The Registrant is registering 1,442,116 shares of its common stock, par
value $.01 per share, reserved for issuance pursuant to the Q-Up Systems, Inc.
1999 Stock Option Plan.

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

<PAGE>   2


                                     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) of the Securities Act
of 1933, as amended (the "Securities Act"). In accordance with the instructions
to Part I of Form S-8, such 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 of the Securities Act. These documents and the documents
incorporated by reference pursuant to Item 3 of Part II of this registration
statement, taken together, constitute the prospectus as required by Section
10(a) of the Securities Act.

                                     PART II

               INFORMATION REQUIRED IN THE REGISTRATION STATEMENT

ITEM 3.   INCORPORATION OF DOCUMENTS BY REFERENCE.

          S1 hereby incorporates by reference into this registration statement
the following documents filed by it with the SEC:

          (a)       S1's annual report on Form 10-K for the 12 months ended
                    December 31, 1999.

          (b)       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.

          (c)       The description of Common Stock 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 and reports filed by S1 subsequent to the
date hereof pursuant to Sections 13(a), 13(c), 14, and 15(d) of the Securities
Exchange Act of 1934, as amended (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 remaining unsold, shall be deemed to be
incorporated by reference in this registration statement and to be part hereof
from the date of filing of such documents or reports. Any statement contained in
a document incorporated or deemed to be incorporated by reference herein shall
be deemed to be modified or superseded for purposes of this registration
statement to the extent that a statement contained herein or in any other
subsequently filed document which also is or is deemed to be incorporated by
reference herein 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.


                                       2
<PAGE>   3


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 provides for indemnification of S1's directors, officers,
employees and agents under certain circumstances.

          S1 also has the power to purchase and maintain insurance on behalf of
its directors, officers, employees and agents and certain other persons. S1 has
in effect a policy of liability insurance covering its directors and officers,
the effect of which is to reimburse its directors and officers against certain
damages and expenses resulting from certain claims made against them caused by
their negligent act, error or omission.

          The foregoing indemnity and insurance provisions have the effect of
reducing directors' and officers' exposure to personal liability for actions
taken in connection with their respective positions.

          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.

<TABLE>
<CAPTION>
      Exhibit No.                          Exhibit
      -----------                          -------
<S>                 <C>
          4.1       Specimen common stock certificate (Incorporated by reference
                    to Exhibit 3.1 to S1's registration statement on Form 8-A
                    (File No. 000-24931) filed with the SEC on September 30,
                    1998.)


          4.2       Specimen certificate for S1's Series A Convertible Preferred
                    Stock (Incorporated by reference to Exhibit 4.2 to S1's
                    Annual Report on Form 10-K for the fiscal year ended
                    December 31, 1998 (File No. 000-24931) filed with the SEC on
                    March 31, 1999.)

          4.3       Specimen certificate for S1's Series B Convertible
                    Redeemable Preferred Stock (Incorporated by reference to
                    Exhibit 4.3 to S1's Annual Report on Form 10-K for the
                    fiscal year ended December 31, 1998 (File No. 000-24931)
                    filed with the SEC on March 31, 1999.)
</TABLE>





                                       3
<PAGE>   4

<TABLE>
        <S>        <C>
          4.4       Specimen certificate for S1's Series C Redeemable
                    Convertible Preferred Stock (Incorporated by reference to
                    Exhibit 4 to S1's Quarterly Report on Form 10-Q for the
                    quarterly period ending March 31, 1999 (File No. 000-24931)
                    filed with the SEC on May 17, 1999.)

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

          23.1      Consent of PricewaterhouseCoopers LLP.

          23.2      Consent of KPMG LLP.

          23.3      Consent of Nancy Kenley, Esq. (included in her opinion filed
                    as Exhibit 5 hereto).

          99.1      Section 145 of the Delaware General Corporation Law
                    (Incorporated by reference to Exhibit 99.1 to S1's
                    Post-Effective Amendment No. 1 to Form S-4 on Form S-8
                    (Registration No. 333-82711) filed with the SEC on January
                    19, 2000).
</TABLE>

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

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:

               (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. Notwithstanding the
          foregoing, any increase or decrease in volume of securities offiered
          (if the total dollar value of securities offered would not exceed that
          which was registered) and any deviation from the low or high end of
          the estimated maximum offering range may be reflected in the form of
          prospectus filed with the Commission pursuant to Rule 424(b) if, in
          the aggregate, the changes in volume and price represent no more than
          20 percent change in the maximum aggregate offering price set forth in
          the "Calculation of Registration Fee" table in the effective
          registration statement.

               (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.



                                       4
<PAGE>   5

                    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 with or furnished to the SEC 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
Securities Exchange Act of 1934 (and, where applicable, each filing of an
employee benefit plan's annual report pursuant to Section 15(d) of the Exchange
Act of 1934) 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.

(h)       Insofar as indemnification for liabilities arising under the
Securities Act of 1933 may be permitted to directors, officers and controlling
persons of the registrant pursuant to the foregoing provisions, or otherwise,
the registrant has been advised that in the opinion of the Securities and
Exchange Commission such indemnification is against public policy as expressed
in the Act and is, therefore, unenforceable. In the event that a claim for
indemnification against such liabilities (other than the payment by the
Registrant or expenses incurred or paid by a director, officer or controlling
person in successful defense of any action, suit or proceeding) is asserted by
such director, officer or controlling person in connection with the securities
being registered, the Rregistrant 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 of 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.



                                       5
<PAGE>   6


                                   SIGNATURES

          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 May 15, 2000.

                                      S1 CORPORATION

                                      By: /s/ James S. Mahan III
                                         ---------------------------------------
                                         James S. Mahan, III
                                         Chief Executive Officer,
                                         President and Director (Principal
                                         Executive 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 on the 15th day of May, 2000.

<TABLE>
<CAPTION>
           SIGNATURE                                                           TITLE
           ---------                                                           -----
<S>                                                         <C>
     /s/ James S. Mahan III                                     Chief Executive Officer, President and
--------------------------------                                Director (Principal Executive Officer)
      James S. Mahan, III

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


                                                                             Director
--------------------------------
       Robert W. Copelan


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

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


--------------------------------                                             Director
    Jackson L. Wilson, Jr.

 /s/ Gregg Freishtat
--------------------------------                                             Director
        Gregg Freishtat
</TABLE>




                                       6
<PAGE>   7



 /s/  Michel Akkermans                                         Director
--------------------------------
       Michel Akkermans

 /s/  David Hodgson                                            Director
--------------------------------
         David Hodgson




                                       7
<PAGE>   8



                                  EXHIBIT INDEX

<TABLE>
<CAPTION>
        Exhibit
          No.                               Exhibit
          ---                               -------
<S>              <C>
          4.1       Specimen common stock certificate (Incorporated by reference
                    to Exhibit 3.1 to S1's registration statement on Form 8-A
                    (File No. 000-24931) filed with the SEC on September 30,
                    1998.)

          4.2       Specimen certificate for S1's Series A Convertible Preferred
                    Stock (Incorporated by reference to Exhibit 4.2 to S1's
                    Annual Report on Form 10-K for the fiscal year ended
                    December 31, 1998 (File No. 000-24931) filed with the SEC on
                    March 31, 1999.)

          4.3       Specimen certificate for S1's Series B Convertible
                    Redeemable Preferred Stock (Incorporated by reference to
                    Exhibit 4.3 to S1's Annual Report on Form 10-K for the
                    fiscal year ended December 31, 1998 (File No. 000-24931)
                    filed with the SEC on March 31, 1999.)

          4.4       Specimen certificate for S1's Series C Redeemable
                    Convertible Preferred Stock (Incorporated by reference to
                    Exhibit 4 to S1's Quarterly Report on Form 10-Q for the
                    quarterly period ending March 31, 1999 (File No. 000-24931)
                    filed with the SEC on May 17, 1999.)

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

          23.1      Consent of PricewaterhouseCoopers LLP.

          23.2      Consent of KPMG LLP

          23.3      Consent of Nancy Kenley, Esq. (included in her opinion filed
                    as Exhibit 5 hereto).

          99.1      Section 145 of the Delaware General Corporation Law
                    (Incorporated by reference to Exhibit to S1's Post-Effective
                    Amendment No. 1 to Form S-4 on Form S-8 (Registration No.
                    333-82711) filed with the SEC on 99.1 January 19, 2000).
</TABLE>
[{"type": "S-8", "sequence": "1", "description": "FORM S-8", "secsgml_size_bytes": 21426}]
{"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": "333-37062", "film-number": "635332"}, "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 MAY 15, 2000
                                                    Registration No. 333-_______
================================================================================

                       SECURITIES AND EXCHANGE COMMISSION
                             WASHINGTON, D.C. 20549
                            ------------------------

                                    FORM S-8
                          REGISTRATION STATEMENT UNDER
                           THE SECURITIES ACT OF 1933

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

                                    DELAWARE
                         (State or other jurisdiction of
                         incorporation or organization)

                                   58-2395199
                      (IRS employer identification number)

                             3390 Peachtree Road, NE
                                   Suite 1700
                             Atlanta, Georgia 30326
                                 (404) 812-6200
               (Address, including zip code, and telephone number,
                 including area code, of registrant's principal
                               executive offices)

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

                    Q-Up Systems, Inc. 1999 Stock Option Plan
                            (Full title of the Plan)

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

                               Robert F. Stockwell
                             Chief Financial Officer
                                 S1 Corporation
                             3390 Peachtree Road, NE
                                   Suite 1700
                             Atlanta, Georgia 30326
                                 (404) 812-6780
            (Name, address and telephone number of Agent for Service)

                                    Copy to:
                              Stuart G. Stein, Esq.
                             Hogan & Hartson L.L.P.
                           555 Thirteenth Street, N.W.
                           Washington, D.C. 20004-1109
                                 (202) 637-8575

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

                         CALCULATION OF REGISTRATION FEE
===========================================================================================================================
                                                AMOUNT         PROPOSED MAXIMUM         PROPOSED MAXIMUM       AMOUNT OF
                  TITLE OF SECURITIES            TO BE          OFFERING PRICE         AGGREGATE OFFERING     REGISTRATION
                    TO BE REGISTERED         REGISTERED(2)      PER SHARE (1)             PRICE (1)(2)         FEE (1)(2)
---------------------------------------------------------------------------------------------------------------------------
Common Stock                                 1,442,116          35.78125                51,600,713.13          13622.59
===========================================================================================================================
(1) Estimated solely for the purpose of calculating the registration fee
pursuant to Rule 457(h) of the Securities Act of 1933, as amended.

(2) The Registrant is registering 1,442,116 shares of its common stock, par
value $.01 per share, reserved for issuance pursuant to the Q-Up Systems, Inc.
1999 Stock Option Plan.

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

                                     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) of the Securities Act
of 1933, as amended (the "Securities Act"). In accordance with the instructions
to Part I of Form S-8, such 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 of the Securities Act. These documents and the documents
incorporated by reference pursuant to Item 3 of Part II of this registration
statement, taken together, constitute the prospectus as required by Section
10(a) of the Securities Act.

                                     PART II

               INFORMATION REQUIRED IN THE REGISTRATION STATEMENT

ITEM 3.   INCORPORATION OF DOCUMENTS BY REFERENCE.

          S1 hereby incorporates by reference into this registration statement
the following documents filed by it with the SEC:

          (a)       S1's annual report on Form 10-K for the 12 months ended
                    December 31, 1999.

          (b)       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.

          (c)       The description of Common Stock 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 and reports filed by S1 subsequent to the
date hereof pursuant to Sections 13(a), 13(c), 14, and 15(d) of the Securities
Exchange Act of 1934, as amended (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 remaining unsold, shall be deemed to be
incorporated by reference in this registration statement and to be part hereof
from the date of filing of such documents or reports. Any statement contained in
a document incorporated or deemed to be incorporated by reference herein shall
be deemed to be modified or superseded for purposes of this registration
statement to the extent that a statement contained herein or in any other
subsequently filed document which also is or is deemed to be incorporated by
reference herein 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.

                                       2
3

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 provides for indemnification of S1's directors, officers,
employees and agents under certain circumstances.

          S1 also has the power to purchase and maintain insurance on behalf of
its directors, officers, employees and agents and certain other persons. S1 has
in effect a policy of liability insurance covering its directors and officers,
the effect of which is to reimburse its directors and officers against certain
damages and expenses resulting from certain claims made against them caused by
their negligent act, error or omission.

          The foregoing indemnity and insurance provisions have the effect of
reducing directors' and officers' exposure to personal liability for actions
taken in connection with their respective positions.

          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.                          Exhibit
      -----------                          -------
4.1       Specimen common stock certificate (Incorporated by reference
                    to Exhibit 3.1 to S1's registration statement on Form 8-A
                    (File No. 000-24931) filed with the SEC on September 30,
                    1998.)

          4.2       Specimen certificate for S1's Series A Convertible Preferred
                    Stock (Incorporated by reference to Exhibit 4.2 to S1's
                    Annual Report on Form 10-K for the fiscal year ended
                    December 31, 1998 (File No. 000-24931) filed with the SEC on
                    March 31, 1999.)

          4.3       Specimen certificate for S1's Series B Convertible
                    Redeemable Preferred Stock (Incorporated by reference to
                    Exhibit 4.3 to S1's Annual Report on Form 10-K for the
                    fiscal year ended December 31, 1998 (File No. 000-24931)
                    filed with the SEC on March 31, 1999.)
3
4
4.4       Specimen certificate for S1's Series C Redeemable
                    Convertible Preferred Stock (Incorporated by reference to
                    Exhibit 4 to S1's Quarterly Report on Form 10-Q for the
                    quarterly period ending March 31, 1999 (File No. 000-24931)
                    filed with the SEC on May 17, 1999.)

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

          23.1      Consent of PricewaterhouseCoopers LLP.

          23.2      Consent of KPMG LLP.

          23.3      Consent of Nancy Kenley, Esq. (included in her opinion filed
                    as Exhibit 5 hereto).

          99.1      Section 145 of the Delaware General Corporation Law
                    (Incorporated by reference to Exhibit 99.1 to S1's
                    Post-Effective Amendment No. 1 to Form S-4 on Form S-8
                    (Registration No. 333-82711) filed with the SEC on January
                    19, 2000).
----------------------------

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:

               (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. Notwithstanding the
          foregoing, any increase or decrease in volume of securities offiered
          (if the total dollar value of securities offered would not exceed that
          which was registered) and any deviation from the low or high end of
          the estimated maximum offering range may be reflected in the form of
          prospectus filed with the Commission pursuant to Rule 424(b) if, in
          the aggregate, the changes in volume and price represent no more than
          20 percent change in the maximum aggregate offering price set forth in
          the "Calculation of Registration Fee" table in the effective
          registration statement.

               (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.

                                       4
5

                    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 with or furnished to the SEC 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
Securities Exchange Act of 1934 (and, where applicable, each filing of an
employee benefit plan's annual report pursuant to Section 15(d) of the Exchange
Act of 1934) 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.

(h)       Insofar as indemnification for liabilities arising under the
Securities Act of 1933 may be permitted to directors, officers and controlling
persons of the registrant pursuant to the foregoing provisions, or otherwise,
the registrant has been advised that in the opinion of the Securities and
Exchange Commission such indemnification is against public policy as expressed
in the Act and is, therefore, unenforceable. In the event that a claim for
indemnification against such liabilities (other than the payment by the
Registrant or expenses incurred or paid by a director, officer or controlling
person in successful defense of any action, suit or proceeding) is asserted by
such director, officer or controlling person in connection with the securities
being registered, the Rregistrant 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 of 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.

                                       5
6

                                   SIGNATURES

          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 May 15, 2000.

                                      S1 CORPORATION

                                      By: /s/ James S. Mahan III
                                         ---------------------------------------
                                         James S. Mahan, III
                                         Chief Executive Officer,
                                         President and Director (Principal
                                         Executive 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 on the 15th day of May, 2000.
SIGNATURE                                                           TITLE
           ---------                                                           -----
/s/ James S. Mahan III                                     Chief Executive Officer, President and
--------------------------------                                Director (Principal Executive Officer)
      James S. Mahan, III

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

                                                                             Director
--------------------------------
       Robert W. Copelan

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

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

--------------------------------                                             Director
    Jackson L. Wilson, Jr.

 /s/ Gregg Freishtat
--------------------------------                                             Director
        Gregg Freishtat
6
7

 /s/  Michel Akkermans                                         Director
--------------------------------
       Michel Akkermans

 /s/  David Hodgson                                            Director
--------------------------------
         David Hodgson

                                       7
8

                                  EXHIBIT INDEX
Exhibit
          No.                               Exhibit
          ---                               -------
4.1       Specimen common stock certificate (Incorporated by reference
                    to Exhibit 3.1 to S1's registration statement on Form 8-A
                    (File No. 000-24931) filed with the SEC on September 30,
                    1998.)

          4.2       Specimen certificate for S1's Series A Convertible Preferred
                    Stock (Incorporated by reference to Exhibit 4.2 to S1's
                    Annual Report on Form 10-K for the fiscal year ended
                    December 31, 1998 (File No. 000-24931) filed with the SEC on
                    March 31, 1999.)

          4.3       Specimen certificate for S1's Series B Convertible
                    Redeemable Preferred Stock (Incorporated by reference to
                    Exhibit 4.3 to S1's Annual Report on Form 10-K for the
                    fiscal year ended December 31, 1998 (File No. 000-24931)
                    filed with the SEC on March 31, 1999.)

          4.4       Specimen certificate for S1's Series C Redeemable
                    Convertible Preferred Stock (Incorporated by reference to
                    Exhibit 4 to S1's Quarterly Report on Form 10-Q for the
                    quarterly period ending March 31, 1999 (File No. 000-24931)
                    filed with the SEC on May 17, 1999.)

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

          23.1      Consent of PricewaterhouseCoopers LLP.

          23.2      Consent of KPMG LLP

          23.3      Consent of Nancy Kenley, Esq. (included in her opinion filed
                    as Exhibit 5 hereto).

          99.1      Section 145 of the Delaware General Corporation Law
                    (Incorporated by reference to Exhibit to S1's Post-Effective
                    Amendment No. 1 to Form S-4 on Form S-8 (Registration No.
                    333-82711) filed with the SEC on 99.1 January 19, 2000).