<PAGE>

            As filed with the Securities and Exchange Commission on May 25, 2001
                                                      Registration No. 333-_____

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

                      SECURITIES AND EXCHANGE COMMISSION
                            WASHINGTON, D.C. 20549

                                   FORM S-8

                         REGISTRATION STATEMENT UNDER
                          THE SECURITIES ACT OF 1933

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

                              AVICI SYSTEMS INC.
            (Exact Name of Registrant as specified in its charter)

            DELAWARE                                             02-0493372
(State or other jurisdiction of                               (I.R.S. Employer
 incorporation or organization)                              Identification No.)

                             101 BILLERICA AVENUE
                           NORTH BILLERICA, MA 01862
                                (978) 964-2000
              (Address of Principal Executive Offices) (Zip Code)

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

                              AVICI SYSTEMS INC.
                     2000 STOCK OPTION AND INCENTIVE PLAN
                           (Full title of the plans)

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

                            PETER C. ANASTOS, ESQ.
                              AVICI SYSTEMS INC.
                             101 BILLERICA AVENUE
                          NORTH BILLERICA, MA  01862
              (Name and Address of Agent for Service of Process)

                                (978) 964-2000
         (Telephone Number, Including Area Code, of Agent For Service)

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

                                   Copy to:

                             JOHN A. MELTAUS, ESQ.
                        TESTA, HURWITZ & THIBEAULT, LLP
                                125 HIGH STREET
                          BOSTON, MASSACHUSETTS 02110
                                (617) 248-7000

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

<PAGE>

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

                        CALCULATION OF REGISTRATION FEE

================================================================================
<TABLE>
<CAPTION>
                                                                 Proposed
                                                                  Maximum        Proposed
                                                                 Offering        Maximum          Amount of
Title of Securities                             Amount to be     Price Per       Aggregate       Registration
to be Registered                                 Registered        Share      Offering Price        Fee(1)
                                                ------------     ---------    --------------     ------------
<S>                                             <C>              <C>           <C>                <C>
2000 STOCK OPTION AND INCENTIVE PLAN
Common Stock (par value $.0001 per share)       5,000,000 (2)     $12.665      $63,325,000        $15,831.25
-------------------------------------------------------------------------------------------------------------
</TABLE>

(1)  Calculated pursuant to Section 6(b) of the Securities Act of 1933, as
     amended.

(2)  The price of $12.665 per share, which is the average of the high and low
     price of the Common Stock of the Registrant as reported on the Nasdaq
     National Market on May 18, 2001, is set forth solely for purposes of
     calculating the filing fee pursuant to Rule 457(c) of the Securities Act
     and has been used only for those shares without a fixed exercise price.

                                       2
<PAGE>

     This Registration Statement registers additional securities of the same
class as other securities for which a Registration Statement on Form S-8 (No.
333-48178) relating to Avici's 2000 Stock Option and Incentive Plan is
effective.  Pursuant to General Instruction E of Form S-8, the contents of the
above-listed Registration Statement are hereby incorporated by reference.

Item 8.  Exhibits.
         --------

Exhibit
  No.    Description
-------  -----------
4.1      Fourth Restated Certificate of Incorporation of the Registrant (filed
         as Exhibit 3.2 to the Registration Statement on Form S-1 (File No. 333-
         37316) and incorporated herein by reference)

4.2      Amended and Restated By-Laws of the Registrant (filed as Exhibit 3.4 to
         the Registration Statement on Form S-1 (File No. 333-37316) and
         incorporated herein by reference)

4.3      2000 Stock Option and Incentive Plan, as amended

5.1      Opinion of Testa, Hurwitz & Thibeault, LLP

23.1     Consent of Testa, Hurwitz & Thibeault, LLP (included in Exhibit 5.1)

23.2     Consent of Arthur Andersen LLP

24.1     Power of Attorney (included as part of the signature page of this
         Registration Statement)


                                       3
<PAGE>

                                  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 Billerica in the State of Massachusetts, on this 25th
day of May, 2001.

                                     AVICI SYSTEMS INC.


                                     By: /s/ Paul F. Brauneis
                                         --------------------
                                         Paul F. Brauneis
                                         Chief Financial Officer and Treasurer

                       POWER OF ATTORNEY AND SIGNATURES

     We, the undersigned officers and directors of Avici Systems Inc., hereby
severally constitute and appoint Paul F. Brauneis, our true and lawful attorney,
with full power to him, to sign for us in our names in the capacities indicated
below, any amendments to this Registration Statement on Form S-8 (including
post-effective amendments), and to file the same, with all exhibits thereto and
other documents in connection therewith, with the Securities and Exchange
Commission, and generally to do all things in our names and on our behalf in our
capacities as officers and directors to enable Avici Systems Inc., to comply
with the provisions of the Securities Act of 1933, as amended, hereby ratifying
and confirming our signatures as they may be signed by our said attorneys, or
any of them, to said Registration Statement and all amendments thereto.

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

       SIGNATURE                                  TITLE

/s/ Surya R. Panditi         Chief Executive Officer, Co-Chairman and Director
-------------------------    (principal executive officer)
Surya R. Panditi

/s/ Steven B. Kaufman        President, Chief Operating Officer and Director
-------------------------
Steven B. Kaufman

/s/ Paul F. Brauneis         Chief Financial Officer and Treasurer (principal
-------------------------    financial officer and principal accounting officer)
Paul F. Brauneis

/s James R. Swartz           Co-Chairman and Director
-------------------------
James R. Swartz

/s/ Stephen M. Diamond       Director
-------------------------
Stephen M. Diamond

/s/ James Mongiello          Director
-------------------------
James Mongiello

/s/ Henry Zannini            Director
-------------------------
Henry Zannini

/s/ Sanjiv Ahuja             Director
-------------------------
Sanjiv Ahuja

                                       4
<PAGE>

                               INDEX TO EXHIBITS

Exhibit
  No.    Description
-------  -----------
4.1      Fourth Restated Certificate of Incorporation of the Registrant (filed
         as Exhibit 3.2 to the Registration Statement on Form S-1 (File No. 333-
         37316) and incorporated herein by reference)

4.2      Amended and Restated By-Laws of the Registrant (filed as Exhibit 3.4 to
         the Registration Statement on Form S-1 (File No. 333-37316) and
         incorporated herein by reference)

4.3      2000 Stock Option and Incentive Plan, as amended

5.1      Opinion of Testa, Hurwitz & Thibeault, LLP

23.1     Consent of Testa, Hurwitz & Thibeault, LLP (included in Exhibit 5.1)

23.2     Consent of Arthur Andersen LLP

24.1     Power of Attorney (included as part of the signature page of this
         Registration Statement)


                                       5
[{"type": "S-8", "sequence": "1", "filename": "ds8.txt", "description": "FORM S-8", "secsgml_size_bytes": 8961}]
{"company-data": {"conformed-name": "AVICI SYSTEMS INC", "cik": "0001094895", "assigned-sic": "3576", "irs-number": "020493372", "state-of-incorporation": "DE", "fiscal-year-end": "1231"}, "filing-values": {"form-type": "S-8", "act": "33", "file-number": "333-61752", "film-number": "1648989"}, "business-address": {"street1": "101 BILLERICA AVENUE", "city": "NORTH BILLERICA", "state": "MA", "zip": "01862", "phone": "9789642000"}, "mail-address": {"street1": "101 BILLERICA AVE", "city": "NORTH BILLERICA", "state": "MA", "zip": "01862"}}
As filed with the Securities and Exchange Commission on May 25, 2001
                                                      Registration No. 333-_____

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

                      SECURITIES AND EXCHANGE COMMISSION
                            WASHINGTON, D.C. 20549

                                   FORM S-8

                         REGISTRATION STATEMENT UNDER
                          THE SECURITIES ACT OF 1933

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

                              AVICI SYSTEMS INC.
            (Exact Name of Registrant as specified in its charter)

            DELAWARE                                             02-0493372
(State or other jurisdiction of                               (I.R.S. Employer
 incorporation or organization)                              Identification No.)

                             101 BILLERICA AVENUE
                           NORTH BILLERICA, MA 01862
                                (978) 964-2000
              (Address of Principal Executive Offices) (Zip Code)

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

                              AVICI SYSTEMS INC.
                     2000 STOCK OPTION AND INCENTIVE PLAN
                           (Full title of the plans)

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

                            PETER C. ANASTOS, ESQ.
                              AVICI SYSTEMS INC.
                             101 BILLERICA AVENUE
                          NORTH BILLERICA, MA  01862
              (Name and Address of Agent for Service of Process)

                                (978) 964-2000
         (Telephone Number, Including Area Code, of Agent For Service)

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

                                   Copy to:

                             JOHN A. MELTAUS, ESQ.
                        TESTA, HURWITZ & THIBEAULT, LLP
                                125 HIGH STREET
                          BOSTON, MASSACHUSETTS 02110
                                (617) 248-7000

--------------------------------------------------------------------------------
================================================================================

                        CALCULATION OF REGISTRATION FEE

================================================================================
Proposed
                                                                  Maximum        Proposed
                                                                 Offering        Maximum          Amount of
Title of Securities                             Amount to be     Price Per       Aggregate       Registration
to be Registered                                 Registered        Share      Offering Price        Fee(1)
                                                ------------     ---------    --------------     ------------
2000 STOCK OPTION AND INCENTIVE PLAN
Common Stock (par value $.0001 per share)       5,000,000 (2)     $12.665      $63,325,000        $15,831.25
-------------------------------------------------------------------------------------------------------------
(1)  Calculated pursuant to Section 6(b) of the Securities Act of 1933, as
     amended.

(2)  The price of $12.665 per share, which is the average of the high and low
     price of the Common Stock of the Registrant as reported on the Nasdaq
     National Market on May 18, 2001, is set forth solely for purposes of
     calculating the filing fee pursuant to Rule 457(c) of the Securities Act
     and has been used only for those shares without a fixed exercise price.

                                       2
This Registration Statement registers additional securities of the same
class as other securities for which a Registration Statement on Form S-8 (No.
333-48178) relating to Avici's 2000 Stock Option and Incentive Plan is
effective.  Pursuant to General Instruction E of Form S-8, the contents of the
above-listed Registration Statement are hereby incorporated by reference.

Item 8.  Exhibits.
         --------

Exhibit
  No.    Description
-------  -----------
4.1      Fourth Restated Certificate of Incorporation of the Registrant (filed
         as Exhibit 3.2 to the Registration Statement on Form S-1 (File No. 333-
         37316) and incorporated herein by reference)

4.2      Amended and Restated By-Laws of the Registrant (filed as Exhibit 3.4 to
         the Registration Statement on Form S-1 (File No. 333-37316) and
         incorporated herein by reference)

4.3      2000 Stock Option and Incentive Plan, as amended

5.1      Opinion of Testa, Hurwitz & Thibeault, LLP

23.1     Consent of Testa, Hurwitz & Thibeault, LLP (included in Exhibit 5.1)

23.2     Consent of Arthur Andersen LLP

24.1     Power of Attorney (included as part of the signature page of this
         Registration Statement)

                                       3
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 Billerica in the State of Massachusetts, on this 25th
day of May, 2001.

                                     AVICI SYSTEMS INC.

                                     By: /s/ Paul F. Brauneis
                                         --------------------
                                         Paul F. Brauneis
                                         Chief Financial Officer and Treasurer

                       POWER OF ATTORNEY AND SIGNATURES

     We, the undersigned officers and directors of Avici Systems Inc., hereby
severally constitute and appoint Paul F. Brauneis, our true and lawful attorney,
with full power to him, to sign for us in our names in the capacities indicated
below, any amendments to this Registration Statement on Form S-8 (including
post-effective amendments), and to file the same, with all exhibits thereto and
other documents in connection therewith, with the Securities and Exchange
Commission, and generally to do all things in our names and on our behalf in our
capacities as officers and directors to enable Avici Systems Inc., to comply
with the provisions of the Securities Act of 1933, as amended, hereby ratifying
and confirming our signatures as they may be signed by our said attorneys, or
any of them, to said Registration Statement and all amendments thereto.

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

       SIGNATURE                                  TITLE

/s/ Surya R. Panditi         Chief Executive Officer, Co-Chairman and Director
-------------------------    (principal executive officer)
Surya R. Panditi

/s/ Steven B. Kaufman        President, Chief Operating Officer and Director
-------------------------
Steven B. Kaufman

/s/ Paul F. Brauneis         Chief Financial Officer and Treasurer (principal
-------------------------    financial officer and principal accounting officer)
Paul F. Brauneis

/s James R. Swartz           Co-Chairman and Director
-------------------------
James R. Swartz

/s/ Stephen M. Diamond       Director
-------------------------
Stephen M. Diamond

/s/ James Mongiello          Director
-------------------------
James Mongiello

/s/ Henry Zannini            Director
-------------------------
Henry Zannini

/s/ Sanjiv Ahuja             Director
-------------------------
Sanjiv Ahuja

                                       4
INDEX TO EXHIBITS

Exhibit
  No.    Description
-------  -----------
4.1      Fourth Restated Certificate of Incorporation of the Registrant (filed
         as Exhibit 3.2 to the Registration Statement on Form S-1 (File No. 333-
         37316) and incorporated herein by reference)

4.2      Amended and Restated By-Laws of the Registrant (filed as Exhibit 3.4 to
         the Registration Statement on Form S-1 (File No. 333-37316) and
         incorporated herein by reference)

4.3      2000 Stock Option and Incentive Plan, as amended

5.1      Opinion of Testa, Hurwitz & Thibeault, LLP

23.1     Consent of Testa, Hurwitz & Thibeault, LLP (included in Exhibit 5.1)

23.2     Consent of Arthur Andersen LLP

24.1     Power of Attorney (included as part of the signature page of this
         Registration Statement)

                                       5