<PAGE>

    As filed with the Securities and Exchange Commission on September 28, 2000
                                                           Registration No. 333-
================================================================================

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

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

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

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

                                   ARIBA, INC.
             (Exact name of registrant as specified in its charter)

<TABLE>
<CAPTION>

<S>                                   <C>                             <C>
            DELAWARE                             7372                     77-0439730
  (State or other jurisdiction       (Primary Standard Industrial        (IRS Employer
of incorporation or organization)     Classification Code Number)     Identification No.)

</TABLE>

                              1565 CHARLESTON ROAD
                         MOUNTAIN VIEW, CALIFORNIA 94043
                                 (650) 930-6200
               (Address of principal executive offices) (Zip Code)

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

                      SUPPLIERMARKET 1999 STOCK OPTION PLAN
                            (Full title of the Plan)

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

                                EDWARD P. KINSEY
  CHIEF FINANCIAL OFFICER, EXECUTIVE VICE PRESIDENT FINANCE AND ADMINISTRATION
                                   ARIBA, INC.
                              1565 CHARLESTON ROAD
                         MOUNTAIN VIEW, CALIFORNIA 94043

                     (Name and address of agent for service)
                                 (650) 930-6200
          (Telephone number, including area code, of agent for service)

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

                         CALCULATION OF REGISTRATION FEE

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

<TABLE>
<CAPTION>

            Title of                                           Proposed Maximum      Proposed Maximum
           Securities                       Amount                 Offering              Aggregate            Amount of
              to be                         to be                    Price               Offering           Registration
           Registered                    Registered (1)            Per Share             Price (2)               Fee
           ----------                    ----------                ---------             -----                   ---
<S>                                      <C>                    <C>                  <C>                    <C>
SUPPLIERMARKET.COM, INC. 1999 STOCK
OPTION PLAN
       Options                             1,700,000 shares                N/A                   N/A                  N/A
       Common Stock (par value $.002)      1,700,000  shares         $45.52 (2)        $77,384,000 (2)         $20,429.39

</TABLE>


(1)      This Registration Statement shall also cover any additional shares of
         Common Stock which become issuable under the SupplierMarket.com, Inc.
         1999 Stock Option Plan and by reason of any stock dividend, stock
         split, recapitalization or other similar transaction effected without
         the receipt of consideration which results in an increase in the number
         of the outstanding shares of Common Stock of Ariba, Inc.

(2)      Calculated solely for purposes of this offering under Rule 457(h) of
         the Securities Act of 1933, as amended, on the basis of the weighted
         average exercise price of the outstanding options.


<PAGE>

                                     PART II

               INFORMATION REQUIRED IN THE REGISTRATION STATEMENT



Item 3.  INCORPORATION OF DOCUMENTS BY REFERENCE

         Ariba, Inc. (the "Registrant") hereby incorporates by reference into
this Registration Statement the following documents previously filed with the
Securities and Exchange Commission (the "SEC"):

         (a)      The Registrant's Annual Report on Form 10-K for the fiscal
                  year ended September 30, 1999;

         (b)      The Registrant's Quarterly Report on Form 10-Q for the fiscal
                  quarter ended June 30, 2000; and

         (c)      The Registrant's Quarterly Report on Form 10-Q for the fiscal
                  quarter ended March 31, 2000; and

         (d)      The Registrant's Quarterly Report on Form 10-Q for the fiscal
                  quarter ended December 31, 1999; and

         (e)      The Registrant's Reports on Form 8-K filed January 25, 2000,
                  March 21, 2000 and July 20, 2000; and

         (f)      The Registrant's Reports on Form 8-K/A filed April 4, 2000 and
                  May 16, 2000; and

         (g)      The Registrant's Registration Statement No. 000-26299 on Form
                  8-A12G filed with the SEC on June 8, 1999, together with all
                  amendments thereto, pursuant to Section 12 of the Securities
                  Exchange Act of 1934, as amended (the "1934 Act") in which
                  there is described the terms, rights and provisions applicable
                  to the Registrant's outstanding Common Stock.

         All reports and definitive proxy or information statements filed
pursuant to Section 13(a), 13(c), 14 or 15(d) of the 1934 Act after the date of
this Registration Statement and prior to the filing of a post-effective
amendment which indicates that all securities offered hereby have been sold or
which deregisters all securities then remaining unsold shall be deemed to be
incorporated by reference into this Registration Statement and to be a part
hereof from the date of filing of such documents.

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 Delaware General Corporation Law authorizes a court
to award or a corporation's board of directors to grant indemnification to
directors and officers in terms sufficiently broad to permit such
indemnification under certain circumstances for liabilities (including
reimbursement for expenses incurred) arising under the 1933 Act. Article VII,
Section 6, of the Registrant's Bylaws provides for mandatory indemnification of
its directors and officers and permissible indemnification of employees and
other agents to the maximum extent permitted by the Delaware General Corporation
Law. The Registrant's Certificate of Incorporation provides that, pursuant to
Delaware law, its directors shall not be liable for monetary damages for breach
of their fiduciary duty as directors to the Registrant and its stockholders.
This provision in the Certificate of Incorporation does not eliminate the
fiduciary duty of the directors, and, in appropriate circumstances, equitable
remedies such as injunctive or other forms of non-monetary relief will remain
available under Delaware law. In addition, each director will continue to be
subject to liability for breach of the director's duty of loyalty to the
Registrant for acts or omissions not in good faith or involving intentional
misconduct, for knowing violations of law, for actions leading to improper
personal


                                      II-1
<PAGE>

benefit to the director and for payment of dividends or approval of stock
repurchases or redemptions that are unlawful under Delaware law. The provision
also does not affect a director's responsibilities under any other law, such as
the federal securities laws or state or federal environmental laws. The
Registrant has entered into Indemnification Agreements with its officers and
directors. The Indemnification Agreements provide the Registrant's officers and
directors with further indemnification to the maximum extent permitted by the
Delaware General Corporation Law.

Item 7.  EXEMPTION FROM REGISTRATION CLAIMED

         Not Applicable.

Item 8.  EXHIBITS

EXHIBIT NUMBER        EXHIBIT

      4               Instrument Defining Rights of Stockholders. Reference is
                      made to Registrant's Registration Statement No. 000-26299
                      on Form 8-A12G, together with all amendments thereto,
                      which is incorporated herein by reference pursuant to Item
                      3(g) of this Registration Statement.

      5               Opinion and consent of Gunderson Dettmer Stough Villeneuve
                      Franklin & Hachigian, LLP.

    23.1              Consent of Independent Auditors.

    23.2              Consent of Gunderson Dettmer Stough Villeneuve Franklin &
                      Hachigian, LLP is contained in Exhibit 5.

    24                Power of Attorney. Reference is made to page II-5 of this
                      Registration Statement.

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 1933 Act, (ii) to reflect in the
prospectus any facts or events arising after the effective date of this
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 this Registration Statement and (iii) to include any
material information with respect to the plan of distribution not previously
disclosed in this Registration Statement or any material change to such
information in this Registration Statement; PROVIDED, however, that clauses
(1)(i) and (1)(ii) shall 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 1934 Act that are incorporated by reference in this
Registration Statement; (2) that for the purpose of determining any liability
under the 1933 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 and (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 Registrant's SupplierMarket.com, Inc. 1999
Stock Option Plan.

                  B.       The undersigned Registrant hereby undertakes that,
for purposes of determining any liability under the 1933 Act, each filing of the
Registrant's annual report pursuant to Section 13(a) or Section 15(d) of the
1934 Act that is incorporated by reference in this 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.       Insofar as indemnification for liabilities arising
under the 1933 Act may be permitted to directors, officers or controlling
persons of the Registrant pursuant to the indemnification provisions summarized
in Item 6 or otherwise, the Registrant has been advised that, in the opinion of
the SEC, such indemnification is against public policy as expressed in the 1933
Act, and is, therefore, unenforceable. In the event that a claim for
indemnification against such liabilities (other than the payment by the
Registrant of expenses incurred or paid by a director, officer or controlling
person of the Registrant 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, the

<PAGE>

Registrant 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 1933 Act and will be governed by the final adjudication of such
issue.

<PAGE>

                                   SIGNATURES



                  Pursuant to the requirements of the Securities Act of 1933,
as amended, 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 Mountain View, State
of California on this 25th day of September, 2000.

                                   ARIBA, INC.


                                   By:  /s/ KEITH J. KRACH
                                        ---------------------
                                        Keith J. Krach
                                        Chairman of the Board, Chief Executive
                                        Officer



                                POWER OF ATTORNEY



KNOW ALL PERSONS BY THESE PRESENTS:

                  That the undersigned officers and directors of Ariba, Inc., a
Delaware corporation, do hereby constitute and appoint Keith J. Krach and Edward
P. Kinsey, and either of them, the lawful attorneys-in-fact and agents with full
power and authority to do any and all acts and things and to execute any and all
instruments which said attorneys and agents, and either one of them, determine
may be necessary or advisable or required to enable said corporation to comply
with the Securities Act of 1933, as amended, and any rules or regulations or
requirements of the Securities and Exchange Commission in connection with this
Registration Statement. Without limiting the generality of the foregoing power
and authority, the powers granted include the power and authority to sign the
names of the undersigned officers and directors in the capacities indicated
below to this Registration Statement, to any and all amendments, both
pre-effective and post-effective, and supplements to this Registration
Statement, and to any and all instruments or documents filed as part of or in
conjunction with this Registration Statement or amendments or supplements
thereof, and each of the undersigned hereby ratifies and confirms all that said
attorneys and agents, or either one of them, shall do or cause to be done by
virtue hereof. This Power of Attorney may be signed in several counterparts.

                  IN WITNESS WHEREOF, each of the undersigned has executed this
Power of Attorney as of the date indicated.

                  Pursuant to the requirements of the Securities Act of 1933, as
amended, this Registration Statement has been signed below by the following
persons in the capacities and on the dates indicated.

<TABLE>
<CAPTION>

Signature                   Title                                              Date
---------                   -----                                              ----
<S>                         <C>                                                <C>
/s/ KEITH J. KRACH
__________________________  Chairman of the Board, Chief Executive Officer      September 25, 2000
      Keith J. Krach        (Principal Executive Officer)

/s/ EDWARD P. KINSEY
__________________________  Chief Financial Officer, Executive Vice
      Edward P. Kinsey      President-Finance and Administration (Principal     September 25, 2000
                            Financial and Accounting Officer) and Secretary
/s/ ROBERT C. KAGLE
__________________________  Director                                            September 25, 2000
      Robert C. Kagle
</TABLE>

                                      II-4
<PAGE>

<TABLE>
<CAPTION>
Signature                                Title                             Date
---------                               -------                           ------
<S>                                   <C>                                 <C>
                                       Director
__________________________
        Paul Hegarty

/s/ JOHN B. MUMFORD
__________________________             Director                           September 25, 2000
       John B. Mumford

/s/ HATIM A. TYABJI
__________________________             Director                           September 25, 2000
       Hatim A. Tyabji

</TABLE>


                                      II-5
<PAGE>

                                  EXHIBIT INDEX



<TABLE>
<CAPTION>
EXHIBIT NUMBER        EXHIBIT
--------------        -------

<S>                   <C>
      5               Opinion and consent of Gunderson Dettmer Stough Villeneuve
                      Franklin & Hachigian, LLP.

    23.1              Consent of Independent Auditors.
</TABLE>


                                      II-6