<PAGE>


    As filed with the Securities and Exchange Commission on October 26, 1999.

                                                     Registration No. 333-______

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

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


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

                                   04-2787865
                      (I.R.S. employer identification no.)

                                  MASSACHUSETTS
         (State or other jurisdiction of incorporation or organization)

                      101 MAIN STREET, CAMBRIDGE, MA, 02142
               (Address of principal executive offices) (Zip Code)

                          1994 LONG-TERM INCENTIVE PLAN
                            (Full title of the plans)

                             ALAN TREFLER, PRESIDENT
                                PEGASYSTEMS INC.
                                 101 MAIN STREET
                               CAMBRIDGE, MA 02142
                     (Name and address of agent for service)

                                 (617) 374-9600
          (Telephone number, including area code, of agent for service)

                         CALCULATION OF REGISTRATION FEE

<TABLE>
<CAPTION>
-----------------------------------------------------------------------------------------------------------------
                                                       Proposed             Proposed
                                                        maximum              maximum                    Amount
  Title of securities            Amount                offering             aggregate                     of
         to be              to be registered           price per             offering                registration
      registered                   (1)                 share (2)            price (2)                     fee
-----------------------------------------------------------------------------------------------------------------
<S>                             <C>                    <C>                  <C>                      <C>
COMMON STOCK,                   4,500,000              $7.84375             $35,296,875                $9,813
$.01 PAR VALUE

-----------------------------------------------------------------------------------------------------------------
</TABLE>


(1)      Plus such additional number of shares as may be required pursuant to
         the plans in the event of a stock dividend, split-up of shares,
         recapitalization or other similar change in the Common Stock.

(2)      Estimated solely for the purpose of calculating the registration fee,
         in accordance with Rule 457(h)(1), on the basis of the average of the
         high and low prices of the Common Stock as reported on the Nasdaq
         National Market System on October 25, 1999.



<PAGE>



                                EXPLANATORY NOTE

         This Registration Statement has been prepared in accordance with the
requirements of Form S-8, as amended, and relates to an aggregate of
4,500,000 shares of Common Stock, $.01 par value per share, of Pegasystems
Inc. (the "Company"), which represent an increase of 2,500,000 in the number
of shares issuable under the Company's 1994 Long-Term Incentive Plan (the
"Plan") approved by the Board of Directors of the Company and subsequently
approved by the stockholders of the Company on May 6, 1998 and an increase of
2,000,000 in the number of shares issuable under the Plan approved by the
Board of Directors on January 18, 1999 and subsequently approved by the
stockholders of the Company on June 21, 1999. The Company previously filed
with the Securities and Exchange Commission on July 31, 1996 a Registration
Statement on Form S-8 (File No. 33-09305), covering an aggregate of 5,750,000
shares of the Company's Common Stock, including 5,000,000 shares then
reserved for issuance under the Plan.

<PAGE>



                                     PART II

               INFORMATION REQUIRED IN THE REGISTRATION STATEMENT


ITEM 3.  INCORPORATION BY REFERENCE.

         The following documents are hereby incorporated by reference in this
Registration Statement:

         (a) The Company's Annual Report on Form 10-K for the year ended
December 31, 1998, as amended by the Company's Form 10-K/A filed on August 13,
1999;

         (b) The Company's Quarterly Report on Form 10-Q for the quarter ended
March 31, 1999, as amended by the Company's Form 10-Q/A filed on August 13,
1999;

         (c) The Company's Current Report on Form 8-K dated June 8, 1999;

         (d) The Company's Quarterly Report on Form 10-Q for the quarter ended
June 30, 1999; and

         (e) The description of the Company's Common Stock incorporated by
reference into the Company's registration statement on Form 8-A filed with the
Commission on June 21, 1996 from the Company's Registration Statement on Form
S-1 initially filed with the Commission on May 15, 1996.

         In addition, all documents filed by the Company after the initial
filing date of this Registration Statement pursuant to Sections 13(a), 13(c), 14
and 15(d) of the Securities Exchange Act of 1934, as amended (the "Exchange
Act"), and prior to the filing of a post-effective amendment which indicates
that all shares registered hereunder have been sold or which de-registers all
shares then remaining unsold, shall be deemed to be incorporated by reference in
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

         The legality of the shares of Common Stock offered hereby has been
passed upon for the Company by Choate, Hall & Stewart, 53 State Street, Exchange
Place, Boston, Massachusetts 02109.



                                      II-1
<PAGE>



ITEM 6.  INDEMNIFICATION OF OFFICERS AND DIRECTORS

         Section 67 of Chapter 156B of the Massachusetts General Laws provides
that a corporation may indemnify its directors and officers to the extent
specified in or authorized by (i) the articles of organization, (ii) a by-law
adopted by the stockholders, or (iii) a vote adopted by the holders of a
majority of the shares of stock entitled to vote on the election of directors.
In all instances, the extent to which a corporation provides indemnification to
its directors and officers under Section 67 is optional. In its Restated
Articles of Organization, the Company has elected to commit to provide
indemnification to its directors and officers in specified circumstances.
Generally, the Restated Articles of Organization provide that the Company shall
indemnify directors and officers of the Company against liabilities and expenses
arising out of legal proceedings brought against them by reason of their status
as directors or officers or by reason of their agreeing to serve, at the request
of the Company, as a director or officer with another organization. Under this
provision, a director or officer of the Company shall be indemnified by the
Company for all costs and expenses (including attorneys' fees), judgments,
liabilities and amounts paid in settlement of such proceedings, even if he is
not successful on the merits, if he acted in good faith in the reasonable belief
that his action was in the best interests of the Company. The Board of Directors
may authorize advancing litigation expenses to a director or officer at his
request upon receipt of an undertaking by any such director or officer to repay
such expenses if it is ultimately determined that he is not entitled to
indemnification for such expenses.

         Article VI of the Company's Restated Articles of Organization
eliminates the personal liability of the Company's directors to the Company or
its stockholders for monetary damages for breach of a director's fiduciary duty,
except for liability (i) for any breach of a director's duty of loyalty to the
Company or its stockholders, (ii) for acts or omissions not in good faith or
which involve intentional misconduct or a knowing violation of laws, (iii) for
authorization of illegal dividends or redemptions, or (iv) for any transaction
in which a director derived an improper personal benefit.

         The Company maintains directors and officers liability insurance for
the benefit of its directors and certain of its officers.

ITEM 7.  EXEMPTION FROM REGISTRATION CLAIMED.

         Not applicable.

ITEM 8.  EXHIBITS.

<TABLE>

         <S>      <C>
         5.1      Opinion of Choate, Hall & Stewart as to the legality of the shares being registered.
         23.1     Consent of Arthur Andersen LLP.
         23.2     Consent of Ernst & Young LLP.
         23.3     Consent of Choate, Hall & Stewart (included in Exhibit 5.1).
         24.1     Power of Attorney (included in page II-3).
         99.1*    The Company's 1994 Long-Term Incentive Plan (as amended and restated).

</TABLE>


-----------------------------------------------------------------------------
*Incorporated by reference to the Company's proxy statement on Schedule 14A (SEC
File No. 001- 11859) filed with the Commission on June 3, 1999.


                                      II-2
<PAGE>





ITEM 9.  UNDERTAKINGS

         (a)      The Company hereby undertakes:

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

                  (2) that, for the purpose of determining any liability under
the Securities Act of 1933, as amended (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; 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 offering.

         (b) The Company hereby undertakes that, for purpose of determining any
liability under the Securities Act, each filing of the Company's annual report
pursuant to Section 13(a) or 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) Insofar as indemnification for liabilities arising under the
Securities Act may be permitted to directors, officers and controlling persons
of the Company pursuant to the foregoing provisions, or otherwise, the Company
has been advised that in the opinion of the Commission 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 the Company of expenses incurred or paid
by a director, officer or controlling person of the Company 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
Company will, unless in the opinion of 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.




                                      II-3
<PAGE>



                                   SIGNATURES

         Pursuant to the requirements of the Securities Act of 1933, the Company
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 Cambridge, The Commonwealth of Massachusetts, on
October 26, 1999.

                                                 PEGASYSTEMS INC.
                                                 (Registrant)



                                                 By:   /s/ Alan Trefler
                                                    ---------------------------
                                                 Alan Trefler
                                                 President



                                      II-4
<PAGE>



                                POWER OF ATTORNEY

         KNOW ALL MEN BY THESE PRESENTS, that each individual whose signature
appears below constitutes and appoints Alan Trefler, Jim O'Halloran and Robert
V. Jahrling, jointly and severally, his true and lawful attorneys-in-fact and
agents with full powers of substitution, for him and in his name, place and
stead, in any and all capacities, to sign any and all amendments (including
post-effective amendments) to this Registration Statement, and to file the same,
with all exhibits thereto, and all documents in connection therewith, with the
Securities and Exchange Commission, granting unto said attorneys-in-fact and
agents, and each of them, full power and authority to do and perform each and
every act and thing requisite and necessary to be in and about the premises, as
fully to all intents and purposes as he might or could do in person, hereby
ratifying and confirming all that said attorneys-in-fact and agents, or any of
them, or their or his substitute or substitutes, may lawfully do or cause to be
done by virtue thereof.

         Pursuant to the requirements of the Securities Act of 1933, this
Registration Statement has been signed below on October 26, 1999 by the
following persons in the capacities indicated.

<TABLE>
<CAPTION>

NAME                                         CAPACITY
----                                         --------
<S>                                          <C>
/s/ Alan Trefler                             President, Clerk and Director
-------------------------------              (PRINCIPAL EXECUTIVE OFFICER)
Alan Trefler


/s/ Jim O'Halloran                           Chief Financial Officer
-------------------------------              (PRINCIPAL FINANCIAL AND ACCOUNTING OFFICER)
Jim O'Halloran


/s/ Steve Kaplan                             Director
------------------------------
Steve Kaplan


/s/ Edward A. Maybury                        Director
------------------------------
Edward A. Maybury


/s/ Edward B. Roberts                        Director
------------------------------
Edward B. Roberts


/s/ Leonard A. Schlesinger                   Director
------------------------------
Leonard A. Schlesinger


</TABLE>


                                      II-5
[{"type": "S-8", "sequence": "1", "description": "S-8", "secsgml_size_bytes": 15443}]
{"company-data": {"conformed-name": "PEGASYSTEMS INC", "cik": "0001013857", "assigned-sic": "7374", "irs-number": "042787865", "fiscal-year-end": "1231"}, "filing-values": {"form-type": "S-8", "act": "33", "file-number": "333-89707", "film-number": "99734111"}, "business-address": {"street1": "101 MAIN ST", "city": "CAMBRIDGE", "state": "MA", "zip": "02142-1590", "phone": "6173749600"}, "mail-address": {"street1": "101 MAIN ST", "city": "CAMBRIDGE", "state": "MA", "zip": "02142-1590"}}
As filed with the Securities and Exchange Commission on October 26, 1999.

                                                     Registration No. 333-______

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

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

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

                                   04-2787865
                      (I.R.S. employer identification no.)

                                  MASSACHUSETTS
         (State or other jurisdiction of incorporation or organization)

                      101 MAIN STREET, CAMBRIDGE, MA, 02142
               (Address of principal executive offices) (Zip Code)

                          1994 LONG-TERM INCENTIVE PLAN
                            (Full title of the plans)

                             ALAN TREFLER, PRESIDENT
                                PEGASYSTEMS INC.
                                 101 MAIN STREET
                               CAMBRIDGE, MA 02142
                     (Name and address of agent for service)

                                 (617) 374-9600
          (Telephone number, including area code, of agent for service)

                         CALCULATION OF REGISTRATION FEE
-----------------------------------------------------------------------------------------------------------------
                                                       Proposed             Proposed
                                                        maximum              maximum                    Amount
  Title of securities            Amount                offering             aggregate                     of
         to be              to be registered           price per             offering                registration
      registered                   (1)                 share (2)            price (2)                     fee
-----------------------------------------------------------------------------------------------------------------
COMMON STOCK,                   4,500,000              $7.84375             $35,296,875                $9,813
$.01 PAR VALUE

-----------------------------------------------------------------------------------------------------------------
(1)      Plus such additional number of shares as may be required pursuant to
         the plans in the event of a stock dividend, split-up of shares,
         recapitalization or other similar change in the Common Stock.

(2)      Estimated solely for the purpose of calculating the registration fee,
         in accordance with Rule 457(h)(1), on the basis of the average of the
         high and low prices of the Common Stock as reported on the Nasdaq
         National Market System on October 25, 1999.
EXPLANATORY NOTE

         This Registration Statement has been prepared in accordance with the
requirements of Form S-8, as amended, and relates to an aggregate of
4,500,000 shares of Common Stock, $.01 par value per share, of Pegasystems
Inc. (the "Company"), which represent an increase of 2,500,000 in the number
of shares issuable under the Company's 1994 Long-Term Incentive Plan (the
"Plan") approved by the Board of Directors of the Company and subsequently
approved by the stockholders of the Company on May 6, 1998 and an increase of
2,000,000 in the number of shares issuable under the Plan approved by the
Board of Directors on January 18, 1999 and subsequently approved by the
stockholders of the Company on June 21, 1999. The Company previously filed
with the Securities and Exchange Commission on July 31, 1996 a Registration
Statement on Form S-8 (File No. 33-09305), covering an aggregate of 5,750,000
shares of the Company's Common Stock, including 5,000,000 shares then
reserved for issuance under the Plan.
PART II

               INFORMATION REQUIRED IN THE REGISTRATION STATEMENT

ITEM 3.  INCORPORATION BY REFERENCE.

         The following documents are hereby incorporated by reference in this
Registration Statement:

         (a) The Company's Annual Report on Form 10-K for the year ended
December 31, 1998, as amended by the Company's Form 10-K/A filed on August 13,
1999;

         (b) The Company's Quarterly Report on Form 10-Q for the quarter ended
March 31, 1999, as amended by the Company's Form 10-Q/A filed on August 13,
1999;

         (c) The Company's Current Report on Form 8-K dated June 8, 1999;

         (d) The Company's Quarterly Report on Form 10-Q for the quarter ended
June 30, 1999; and

         (e) The description of the Company's Common Stock incorporated by
reference into the Company's registration statement on Form 8-A filed with the
Commission on June 21, 1996 from the Company's Registration Statement on Form
S-1 initially filed with the Commission on May 15, 1996.

         In addition, all documents filed by the Company after the initial
filing date of this Registration Statement pursuant to Sections 13(a), 13(c), 14
and 15(d) of the Securities Exchange Act of 1934, as amended (the "Exchange
Act"), and prior to the filing of a post-effective amendment which indicates
that all shares registered hereunder have been sold or which de-registers all
shares then remaining unsold, shall be deemed to be incorporated by reference in
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

         The legality of the shares of Common Stock offered hereby has been
passed upon for the Company by Choate, Hall & Stewart, 53 State Street, Exchange
Place, Boston, Massachusetts 02109.

                                      II-1
ITEM 6.  INDEMNIFICATION OF OFFICERS AND DIRECTORS

         Section 67 of Chapter 156B of the Massachusetts General Laws provides
that a corporation may indemnify its directors and officers to the extent
specified in or authorized by (i) the articles of organization, (ii) a by-law
adopted by the stockholders, or (iii) a vote adopted by the holders of a
majority of the shares of stock entitled to vote on the election of directors.
In all instances, the extent to which a corporation provides indemnification to
its directors and officers under Section 67 is optional. In its Restated
Articles of Organization, the Company has elected to commit to provide
indemnification to its directors and officers in specified circumstances.
Generally, the Restated Articles of Organization provide that the Company shall
indemnify directors and officers of the Company against liabilities and expenses
arising out of legal proceedings brought against them by reason of their status
as directors or officers or by reason of their agreeing to serve, at the request
of the Company, as a director or officer with another organization. Under this
provision, a director or officer of the Company shall be indemnified by the
Company for all costs and expenses (including attorneys' fees), judgments,
liabilities and amounts paid in settlement of such proceedings, even if he is
not successful on the merits, if he acted in good faith in the reasonable belief
that his action was in the best interests of the Company. The Board of Directors
may authorize advancing litigation expenses to a director or officer at his
request upon receipt of an undertaking by any such director or officer to repay
such expenses if it is ultimately determined that he is not entitled to
indemnification for such expenses.

         Article VI of the Company's Restated Articles of Organization
eliminates the personal liability of the Company's directors to the Company or
its stockholders for monetary damages for breach of a director's fiduciary duty,
except for liability (i) for any breach of a director's duty of loyalty to the
Company or its stockholders, (ii) for acts or omissions not in good faith or
which involve intentional misconduct or a knowing violation of laws, (iii) for
authorization of illegal dividends or redemptions, or (iv) for any transaction
in which a director derived an improper personal benefit.

         The Company maintains directors and officers liability insurance for
the benefit of its directors and certain of its officers.

ITEM 7.  EXEMPTION FROM REGISTRATION CLAIMED.

         Not applicable.

ITEM 8.  EXHIBITS.
5.1      Opinion of Choate, Hall & Stewart as to the legality of the shares being registered.
         23.1     Consent of Arthur Andersen LLP.
         23.2     Consent of Ernst & Young LLP.
         23.3     Consent of Choate, Hall & Stewart (included in Exhibit 5.1).
         24.1     Power of Attorney (included in page II-3).
         99.1*    The Company's 1994 Long-Term Incentive Plan (as amended and restated).
-----------------------------------------------------------------------------
*Incorporated by reference to the Company's proxy statement on Schedule 14A (SEC
File No. 001- 11859) filed with the Commission on June 3, 1999.

                                      II-2
ITEM 9.  UNDERTAKINGS

         (a)      The Company hereby undertakes:

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

                  (2) that, for the purpose of determining any liability under
the Securities Act of 1933, as amended (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; 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 offering.

         (b) The Company hereby undertakes that, for purpose of determining any
liability under the Securities Act, each filing of the Company's annual report
pursuant to Section 13(a) or 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) Insofar as indemnification for liabilities arising under the
Securities Act may be permitted to directors, officers and controlling persons
of the Company pursuant to the foregoing provisions, or otherwise, the Company
has been advised that in the opinion of the Commission 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 the Company of expenses incurred or paid
by a director, officer or controlling person of the Company 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
Company will, unless in the opinion of 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.

                                      II-3
SIGNATURES

         Pursuant to the requirements of the Securities Act of 1933, the Company
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 Cambridge, The Commonwealth of Massachusetts, on
October 26, 1999.

                                                 PEGASYSTEMS INC.
                                                 (Registrant)

                                                 By:   /s/ Alan Trefler
                                                    ---------------------------
                                                 Alan Trefler
                                                 President

                                      II-4
POWER OF ATTORNEY

         KNOW ALL MEN BY THESE PRESENTS, that each individual whose signature
appears below constitutes and appoints Alan Trefler, Jim O'Halloran and Robert
V. Jahrling, jointly and severally, his true and lawful attorneys-in-fact and
agents with full powers of substitution, for him and in his name, place and
stead, in any and all capacities, to sign any and all amendments (including
post-effective amendments) to this Registration Statement, and to file the same,
with all exhibits thereto, and all documents in connection therewith, with the
Securities and Exchange Commission, granting unto said attorneys-in-fact and
agents, and each of them, full power and authority to do and perform each and
every act and thing requisite and necessary to be in and about the premises, as
fully to all intents and purposes as he might or could do in person, hereby
ratifying and confirming all that said attorneys-in-fact and agents, or any of
them, or their or his substitute or substitutes, may lawfully do or cause to be
done by virtue thereof.

         Pursuant to the requirements of the Securities Act of 1933, this
Registration Statement has been signed below on October 26, 1999 by the
following persons in the capacities indicated.
NAME                                         CAPACITY
----                                         --------
/s/ Alan Trefler                             President, Clerk and Director
-------------------------------              (PRINCIPAL EXECUTIVE OFFICER)
Alan Trefler

/s/ Jim O'Halloran                           Chief Financial Officer
-------------------------------              (PRINCIPAL FINANCIAL AND ACCOUNTING OFFICER)
Jim O'Halloran

/s/ Steve Kaplan                             Director
------------------------------
Steve Kaplan

/s/ Edward A. Maybury                        Director
------------------------------
Edward A. Maybury

/s/ Edward B. Roberts                        Director
------------------------------
Edward B. Roberts

/s/ Leonard A. Schlesinger                   Director
------------------------------
Leonard A. Schlesinger
II-5