PROSPECTUS


                                SIS BANCORP, INC.
                         146,400 Shares of Common Stock
                            PAR VALUE $0.01 PER SHARE

         This  Prospectus  relates to 146,400  shares (the  "Shares")  of common
stock,  par value $0.01 per share ("Common  Stock"),  of SIS Bancorp,  Inc. (the
"Company").  Some or all of the Shares will be sold prior to the consummation of
the Company's  pending  acquisition  (the  "Acquisition")  of Glastonbury Bank &
Trust Company, a  Connecticut-chartered  commercial bank located in Glastonbury,
Connecticut ("GBT"),  which is the subject of a separate registration  statement
on Form S-4 filed by the Company.  It is anticipated that all of the Shares sold
will be sold at prices and on terms then available in brokers' transactions. The
exact  number  of  Shares  to be  sold  will  be  determined  only  after  GBT's
stockholders have voted upon the Acquisition, and will depend upon the extent to
which GBT stockholders exercise dissenters' rights of appraisal.  It is expected
that approximately 13,000 Shares will be sold, but if a sufficient number of GBT
stockholders  exercise dissenters' rights, the number of Shares sold could be as
high as 146,400.

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


THESE  SECURITIES  HAVE NOT BEEN APPROVED OR  DISAPPROVED  BY THE SECURITIES AND
EXCHANGE  COMMISSION OR ANY STATE  SECURITIES  COMMISSION NOR HAS THE SECURITIES
AND  EXCHANGE  COMMISSION  OR ANY STATE  SECURITIES  COMMISSION  PASSED UPON THE
ACCURACY OR ADEQUACY OF THIS PROSPECTUS. ANY REPRESENTATION TO THE CONTRARY IS A
CRIMINAL OFFENSE.

THE  SECURITIES  OFFERED  HEREBY ARE NOT  SAVINGS  ACCOUNTS,  DEPOSITS  OR OTHER
OBLIGATIONS  OF ANY  BANK OR  NON-BANK  SUBSIDIARY  OF THE  COMPANY  AND ARE NOT
INSURED BY THE FEDERAL DEPOSIT INSURANCE CORPORATION, THE BANK INSURANCE FUND OR
ANY OTHER GOVERNMENT AGENCY.

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

                                                              Net Proceeds to
                      Price to Public    Offering Expenses        Company
Per Share (1)......       $33.25              $0.97              $419,640
Total (2)............     $33.25              $0.97              $419,640

(1)      Represents  only 13,000  shares being sold in the present  transaction.
         Additional shares may be sold subsequently at prices different than the
         price at which the shares in the present transaction are being sold.

(2)      Includes only shares offered in the present  transaction and any shares
         previously  sold as part of this  offering,  as  described in note (1).
         Does not include the unsold  balance of 133,400  shares covered by this
         registration statement of which this prospectus forms a part.

                The date of this Prospectus is December 1, 1997.





                              AVAILABLE INFORMATION

         The Company has filed with the Securities and Exchange  Commission (the
"Commission")  in  Washington,  D.C.,  a  registration  statement  on  Form  S-3
(together with all exhibits, schedules and amendments thereto, the "Registration
Statement") under the Securities Act of 1933, as amended (the "Securities Act"),
with respect to the Shares. This Prospectus, which is a part of the Registration
Statement, does not contain all of the information set forth in the Registration
Statement.  Statements in this  Prospectus as to the contents of any contract or
other document are not necessarily  complete,  and in each instance reference is
made to the copy of such contract or other  document  filed as an exhibit to the
Registration  Statement,  each such statement being qualified in all respects by
such reference and the exhibits and schedules thereto.  For further  information
concerning  the Company and the Shares,  reference  is made to the  Registration
Statement.  Copies  of the  Registration  Statement  may be  obtained  from  the
Commission  at its  principal  office in  Washington,  D.C.  upon payment of the
prescribed fee.

         The  Company  is  subject  to  the  informational  requirements  of the
Securities  Exchange  Act of 1934,  as amended  (the  "Exchange  Act"),  and, in
accordance  therewith,  files reports and other information with the Commission.
The Registration  Statement,  the exhibits and schedules  forming a part thereof
and the reports,  proxy  statements and other  information  filed by the Company
with  the  Commission  can be  inspected  and  copied  at the  public  reference
facilities maintained by the Commission at Judiciary Plaza, Room 1024, 450 Fifth
Street, N.W.,  Washington,  D.C. 20549, and at the following regional offices of
the Commission:  Chicago Regional  Office,  Suite 1400, 500 West Madison Street,
Chicago,  Illinois  60661-2511;  and New York Regional Office, Seven World Trade
Center,  New York,  New York 10048.  Copies of such  material can be obtained at
prescribed  rates from the Public  Reference  Section of the  Commission  at its
principal office at 450 Fifth Street, N.W., Washington, D.C. 20549. In addition,
reports,  proxy  materials and other  information  concerning the Company may be
inspected at the offices of the NASD, Inc. at 1735 K Street,  N.W.,  Washington,
D.C.  20006.  The  Commission  also  maintains a World Wide Web site (located at
http://www.sec.gov)  that contains reports, proxy and information statements and
other information regarding the Company.

                 INCORPORATION OF CERTAIN DOCUMENTS BY REFERENCE

         The  following  documents,  which have been  filed with the  Commission
pursuant to the Exchange Act, are hereby  incorporated  in this  Prospectus  and
specifically made a part hereof by reference: (i) the Company's Annual Report on
Form 10-K for the fiscal  year  ended  December  31,  1996;  (ii) the  Company's
Quarterly  Reports on Form 10-Q for the quarters ended March 31, 1997,  June 30,
1997 and September 30, 1997;  (iii) the  Company's  Current  Report on Form 8-K,
dated August 18, 1997; and (iv) the Company's Registration Statement on Form 8-A
dated January 23, 1997. All documents  filed by the Company  pursuant to Section
13(a),  13(c),  14 or 15(d) of the Exchange Act  subsequent  to the date of this
Prospectus  and prior to the  termination of the offering of the Shares shall be
deemed to be  incorporated  by reference  into this  Prospectus and to be a part
hereof from the  respective  dates of filing of such  documents.  Any  statement
contained  herein or in a  document  incorporated  or deemed to be  incorporated
herein by reference shall be deemed to be modified or superseded for purposes of
this  Prospectus  to the extent  that a  statement  contained  herein (or in the
applicable Prospectus  Supplement),  or in any other subsequently filed document
that also is or is deemed to be  incorporated  herein by reference,  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
Prospectus.

         The Company  will  provide  without  charge to each person to whom this
Prospectus is delivered, upon the written or oral request of such person, a copy
of any and all of the  information  that has been  incorporated  by reference in
this  Prospectus  (excluding  exhibits  unless such  exhibits  are  specifically
incorporated   by  reference   into  the   information   that  this   Prospectus
incorporates).  Requests  for such  copies  should be made to the Company at its
principal executive offices, 1441 Main Street, Springfield, Massachusetts 01102,
Attention: Clerk, telephone (413) 748-8000.


                                       -2-





                                   THE COMPANY

         The Company is a  Massachusetts  corporation  organized in 1996 for the
purpose of becoming the holding  company of Springfield  Institution for Savings
(the "Bank"), a wholly-owned  subsidiary of the Company. As of June 30, 1997 the
Company had total assets of $1.4 billion,  total  deposits of $1.0 billion,  net
loans of $645.9 million, and stockholders' equity of $103.2 million.

         The Bank was established in 1827 as a state  chartered,  mutual savings
bank headquartered in Springfield, Massachusetts. The Bank converted from mutual
to stock  form in  1995.  The  Company  provides  a wide  variety  of  financial
services,   including  retail  and  commercial  banking,   residential  mortgage
origination and servicing,  commercial and industrial  lending,  commercial real
estate  lending  and  consumer  lending.  Substantially  all  of  the  Company's
operations  are  conducted  through the Bank.  The Company  services its primary
market  of  Hampden  and  Hampshire  Counties  through  a  network  of 25 retail
branches.

         The principal executive offices of the Company and the Bank are located
at 1441 Main Street,  Springfield,  Massachusetts  01102. The Company  telephone
number is (413) 748-8000.

         The Company  and the Bank are subject to federal,  state and local laws
applicable  to  state  savings  banks  and  bank  holding  companies  and to the
regulations of the Board of Governors of the Federal Reserve System, the Federal
Deposit Insurance Corporation and the Massachusetts Division of Banks.

                                 USE OF PROCEEDS

         The net  proceeds  to the  Company  from the sale of the Shares will be
approximately $440,000,  after deducting the estimated offering expenses, if the
anticipated  number of Shares is sold. If a greater  number Shares is sold,  the
net proceeds to the Company will be proportionately greater. The Company intends
to use  the net  proceeds  for  working  capital  needs  and  general  corporate
purposes.

                              PLAN OF DISTRIBUTION

         Some or all of the Shares will be sold prior to the consummation of the
Acquisition,  which, as discussed in a separate  registration  statement on Form
S-4 concerning the planned issuance of Common Stock in the Acquisition, is to be
accounted for as a pooling of interests. The Shares are being sold to reduce the
Company's  treasury  shares to a level that will  qualify  the  Acquisition  for
pooling treatment under applicable  accounting rules. The exact number of Shares
to be sold will depend upon the extent to which GBT stockholders dissenting from
the  Acquisition  choose to assert  appraisal  rights-if  appraisal  rights  are
asserted in respect of more shares,  a greater number of Shares will be sold. If
no dissenters'  rights are asserted,  approximately  13,000 Shares will be sold,
but if sufficient GBT  stockholders  assert  dissenters'  rights,  the number of
Shares sold could be as high as 146,400.

         It is  anticipated  that all of the Shares  sold will be sold at prices
and on terms  then  available  in  brokers'  transactions.  Dealers  or  brokers
participating  in such  transactions  may act as agent for the  Company,  or may
purchase the Shares offered hereby from the Company as principal and resell such
Shares from time to time in or through  transactions or  distributions at market
prices prevailing at the time of sale. The dealers or brokers who participate in
the sale or  distribution of such Shares may be deemed to be  "underwriters"  as
defined in the Securities Act. Any distributors' or sellers' commissions paid or
allowed to any such participating  dealers or brokers,  and, if any such dealers
or  brokers  purchase  shares  as  principal,   any  distributors'  or  sellers'
commissions or profits  received on the resale of such Shares,  may be deemed to
be discounts and commissions  under the Securities Act. All costs,  expenses and
fees incurred in connection with the  registration of the Shares are being borne
by the Company.


                                       -3-





                          DESCRIPTION OF CAPITAL STOCK

Common Stock

         General.  As of October  10,  1997,  the  capital  stock of the Company
consisted  of  25,000,000  authorized  shares,  par  value  of $.01  per  share,
20,000,000 of which are shares of Common Stock,  of which  5,580,842 were issued
and  outstanding  (exclusive  of treasury  shares),  and  5,000,000 of which are
shares  of  preferred  stock  (the  "Preferred   Stock"),   none  of  which  are
outstanding.  The Common Stock is traded on the Nasdaq National Market under the
trading symbol "SISB."

         Shares of the  Common  Stock may be issued  from time to time,  in such
amount and proportions and for such  consideration  as may be fixed by the Board
of Directors of the Company. No holder of the Common Stock has any preemptive or
preferential  rights to purchase or to subscribe for any shares of capital stock
or other securities which may be issued by the Company.  The Common Stock has no
redemption or sinking fund provisions  applicable  thereto and has no conversion
rights.

         The  outstanding  shares  of  the  Common  Stock  are  fully  paid  and
nonassessable.

         Liquidation. In the event of any liquidation, dissolution or winding up
of the  Company,  whether  voluntary or  involuntary,  the holders of the Common
Stock are entitled to receive,  on a share-for-share  basis, any assets or funds
of the Company which are  distributable  to the holders of the Common Stock upon
such  events,  subject to the prior  rights of  creditors of the Company and the
holders of outstanding shares of the Preferred Stock, if any.

         Voting.  The holders of the Common  Stock are  entitled to one vote for
each share in all matters  voted upon by the  stockholders  of the Company.  The
shares of the Common Stock have noncumulative voting rights;  consequently,  the
holders of a majority in interest of the Common Stock can conceivably  elect all
of the directors of the Company and, in such event, the holders of the remaining
shares voting for election of directors would not be able to elect any person or
persons to the Board of Directors of the Company.

         Dividends.  When and if  dividends,  payable  as  cash,  stock or other
property,  are  declared by the Board of  Directors  of the Company out of funds
legally  available  therefor,  the holders of the Common  Stock are  entitled to
share equally,  share for share, in such dividends.  The payment of dividends on
the Common Stock may be subject to applicable bank regulatory approval.

         Preferred  Share  Purchase  Rights.  On January 22, 1997, the Company's
Board of Directors authorized the issuance of one Preferred Share Purchase Right
(a "Right") for each outstanding  share of the Common Stock pursuant to a Rights
Agreement,  dated as of January 22,  1997,  between the Company and  ChaseMellon
Shareholder  Services,  L.L.C.,  as Rights Agent (the "Rights  Agreement").  The
issuance  was made to  stockholders  of record as of the  close of  business  on
February 3, 1997.  Holders of the Common  Stock issued  subsequent  to that date
automatically  receive the Rights with their  shares.  Each Right  entitles  the
holder thereof to purchase under certain  circumstances  one  one-hundredth of a
share of Series A Junior  Participating  Preferred  Stock,  par  value  $.01 per
share, or, in certain  circumstances,  to receive cash, property,  shares of the
Common Stock or other  securities of the Company at a purchase  price of $100.00
per one-hundredth of a preferred share (the "Purchase Price").

         The Rights automatically attach to all certificates representing shares
of the Common Stock and no separate Rights  certificates  have been distributed.
The  Rights  would   separate  from  the  shares  of  the  Common  Stock  and  a
"Distribution  Date" would  occur upon the  earlier of (i) 10 business  days (or
such later date as the  Company's  Board of  Directors  may  determine  before a
Distribution Date occurs) following a public  announcement by the Company that a
person or group of affiliated or associated persons, with certain exceptions (an
"Acquiring  Person"),  has  acquired,  or has  obtained  the  right to  acquire,
beneficial  ownership  of 10% or more of the  outstanding  shares of the  Common
Stock (the date of such announcement being the "Stock Acquisition Date") or (ii)
10 business days

                                       -4-





(or such later date as the Company's  Board of Directors may determine  before a
Distribution  Date  occurs)  following  the  commencement  of a tender  offer or
exchange offer that would result in a person becoming an Acquiring Person.

         Until a  Distribution  Date  occurs,  the Rights are  evidenced  by the
certificates  for shares of the Common Stock and are  transferred  with and only
with such Common Stock certificates,  and the surrender for transfer of any such
certificates  also  constitutes  the transfer of the Rights  associated with the
shares  represented  by  such  certificates.  As  soon  as  practicable  after a
Distribution  Date, Rights  certificates would be mailed to holders of record of
shares of the Common Stock as of the close of business on the Distribution  Date
and, from and after the Distribution  Date,  separate Rights  Certificates alone
would represent the Rights.

         The  Rights  are not  exercisable  until a  Distribution  Date and will
expire at the close of business on January 22, 2007,  unless earlier redeemed or
exchanged by the Company as described below.

         In the event (a "Flip-In  Event")  that a person  becomes an  Acquiring
Person (except pursuant to a tender or exchange offer for all outstanding shares
of the Common  Stock at a price and on terms which a majority  of the  Company's
Outside Directors (as defined in the Rights Agreement)  determines to be fair to
and otherwise in the best interests of the Company and its shareholders (a "fair
offer")),  each  holder of a Right  thereafter  has the right to  receive,  upon
exercise   of  such  Right,   shares  of  the  Common   Stock  (or,  in  certain
circumstances,  cash,  property or other  securities  of the  Company)  having a
Current Market Price (as defined in the Rights Agreement) equal to two times the
exercise  price of the  Right.  Notwithstanding  the  foregoing,  following  the
occurrence  of any  Flip-In  Event,  all  Rights  that are,  or  (under  certain
circumstances specified in the Rights Agreement) were, beneficially owned by any
Acquiring  Person (or by certain  related  parties)  become null and void in the
circumstances  set  forth  in the  Right  Agreement.  However,  Rights  are  not
exercisable following the occurrence of any Flip-In Event until such time as the
Rights are no longer redeemable by the Company as described below.

         In the event (a  "Flip-Over  Event")  that, at any time on or after the
Stock  Acquisition  Date,  (i) the Company  shall take part in a merger or other
business combination  transaction (other than certain mergers that follow a fair
offer) and the  Company  shall not be the  surviving  entity or (ii) the Company
shall take part in a merger or other business  combination  transaction in which
the shares of the Common  Stock are changed or  exchanged  (other  than  certain
mergers that follow a fair offer) or (iii) 50% or more of the  Company's  assets
or earning power is sold or  transferred,  each holder of a Right (except Rights
which previously have been voided,  as described above) thereafter has the right
to receive,  upon exercise,  a number of shares of common stock of the acquiring
company  having a Current  Market Price equal to two times the exercise price of
the Right.

         The Purchase  Price payable and the number of shares of Series A Junior
Participating  Preferred  Stock  (or the  amount  of  cash,  property  or  other
securities)  issuable upon exercise of the Rights are subject to adjustment from
time to time to prevent  dilution (i) in the event of a share  dividend on, or a
subdivision,  combination or reclassification  of, the shares of Series A Junior
Participating  Preferred Stock, (ii) if holders of the shares of Series A Junior
Participating  Preferred  Stock  are  granted  certain  rights  or  warrants  to
subscribe  for  shares  of  Series A  Junior  Participating  Preferred  Stock or
convertible  securities  at less than the Current  Market  Price of the Series A
Junior  Participating  Preferred Stock or (iii) upon the distribution to holders
of shares of the Series A Junior  Participating  Preferred Stock of evidences of
indebtedness  or assets  (excluding  regular  quarterly  cash  dividends)  or of
subscription rights or warrants (other than those referred to above).

         At any time until 10 business days following a Stock  Acquisition Date,
the Company may redeem the Rights in whole,  but not in part, at a price of $.01
per Right,  payable, at the option of the Company, in cash, shares of the Common
Stock or other  consideration as the Company's Board of Directors may determine.
Immediately  upon the  effectiveness  of the  action of the  Company's  Board of
Directors  ordering  redemption of the Rights, the Rights will terminate and the
only  right of the  holders  of  Rights  will be to  receive  the $.01 per Right
redemption price.


                                       -5-





         The terms of the Rights, other than key financial terms and the date on
which the Rights  expire,  may be amended by the  Company's  Board of  Directors
prior to a Distribution Date. Thereafter, the provisions of the Rights Agreement
may be amended by the Board of  Directors  only in order to cure any  ambiguity,
defect or  inconsistency,  to make  changes  which do not  adversely  affect the
interests of holders of Rights  (excluding the interests of any Acquiring Person
and certain  other  related  parties) or to shorten or lengthen  any time period
under the Rights Agreement; provided, however, that no amendment to lengthen the
time period  governing  redemption  shall be made at such time as the Rights are
not redeemable.

         The foregoing description of the Rights does not purport to be complete
and is  qualified  in its  entirety  by  reference  to the Rights  Agreement  as
included  as Exhibit 4.1 to the  Company's  Registration  Statement  on Form 8-A
dated January 23, 1997.

Preferred Stock

         Under the Company's  Articles of  Organization,  the Company's Board of
Directors is authorized,  without further stockholder action, to provide for the
issuance of Preferred  Stock in one or more series,  with such  designations  or
titles,  dividend rates, special or relative rights in the event of liquidation,
distribution  or sale of assets or  dissolution  or winding  up of the  Company,
sinking fund provisions,  redemption or purchase account provisions,  conversion
provisions,  and voting rights as shall be set forth as and when  established by
the Company's Board of Directors.

                                  LEGAL MATTERS

         Certain  legal  matters  with respect to the Shares will be passed upon
for the Company by Sullivan & Worcester LLP, Boston, Massachusetts.


                                     EXPERTS

         The consolidated  financial statements  incorporated in this Prospectus
by  reference  to the  Company's  Annual  Report on Form 10-K for the year ended
December 31, 1996 have been so  incorporated  in reliance on the report of Price
Waterhouse LLP, independent accountants,  given on the authority of said firm as
experts in auditing and accounting.



                                       -6-




         No dealer, sales person or other individual has been authorized to give
any information or make any  representations  not contained in the Prospectus in
connection with the offering covered by this Prospectus.  If given or made, such
information or representations must not be relied upon as having been authorized
by the  Company.  This  Prospectus  does  not  constitute  an offer to sell or a
solicitation  of an offer  to buy,  any  securities  other  than the  registered
securities to which it relates in any  jurisdiction  where,  or to any person to
whom, it is unlawful to make such offer or solicitation. Neither the delivery of
this  Prospectus  or any sale made  hereunder  shall,  under any  circumstances,
create an implication  that there has not been any change in the facts set forth
in this Prospectus or in the affairs of the Company since the date hereof.


                  TABLE OF CONTENTS


                                                 Page

Available Information...........................   2
Incorporation of Certain Documents
     by Reference...............................   2
The Company ....................................   3
Use of Proceeds.................................   3
Plan of Distribution............................   3
Description of Capital Stock ...................   4
Legal Matters...................................   6
Experts.........................................   6





                                       SIS
                                    BANCORP,
                                      INC.


                         146,400 Shares of Common Stock
                           (par value $0.01 per share)


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

                                   PROSPECTUS

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





                                December 1, 1997