<PAGE>
 
                                                                       EXHIBIT 5
                                                                       ---------



                               HALE AND DORR LLP
                              Counsellors at Law

                 60 State Street, Boston, Massachusetts 02109
                         617-526-6000/Fax 617-526-5000



                                 May 14, 1999

Banyan Systems Incorporated
120 Flanders Road
Westboro, Massachusetts 01581

          Re:  1992 Stock Incentive Plan
               -------------------------

Ladies and Gentlemen:

          We have assisted in the preparation of a Registration Statement on
Form S-8 (the "Registration Statement") to be filed with the Securities and
Exchange Commission (the "Commission") relating to 950,000 additional shares of
Common Stock, $.01 par value per share (the "Shares"), of Banyan Systems
Incorporated, a Massachusetts corporation (the "Company"), issuable pursuant to
the Company's 1992 Stock Incentive Plan (the "Plan").

          We have examined the Second Amended and Restated Articles of
Organization of the Company, as amended, the By-Laws of the Company, as amended,
and originals, or copies certified to our satisfaction, of all pertinent records
of the meetings of the directors and stockholders of the Company, the
Registration Statement and such other documents relating to the Company as we
have deemed material for the purposes of this opinion.

          In examination of the foregoing documents, we have assumed the
genuineness of all signatures and the authenticity of all documents submitted to
us as originals, the conformity to original documents of all documents submitted
to us as certified or photostatic copies, and the authenticity of the originals
of such latter documents.

          We assume that the appropriate action will be taken, prior to the
offer and the sale of the Shares issuable pursuant to the Plan, to register and
qualify the Shares for sale under all applicable state securities or "blue sky"
laws.
<PAGE>
 
Banyan Systems Incorporated
May 14, 1999
Page 2

We express no opinion herein as to the laws of any state or jurisdiction other
than the state laws of the Commonwealth of Massachusetts and the federal laws of
the United States of America.

          Based upon and subject to the foregoing, we are of the opinion that
the Company has duly authorized for issuance the Shares and such Shares, when
issued and paid for in accordance with the terms of the Plan and for a
consideration in excess of par value per share, will be validly issued, fully
paid and nonassessable.

          It is understood that this opinion is to be used only in connection
with the offer and sale of the Shares while the Registration Statement is in
effect.

          Please note that we are opining only as to the matters expressly set
forth herein, and no opinion should be inferred as to any other matters.

          We hereby consent to the filing of this opinion with the Commission as
an exhibit to the Registration Statement in accordance with the requirements of
Item 601(b)(5) of Regulation S-K under the Securities Act of 1933, as amended
(the "Securities Act").  In giving such consent, we do not hereby admit that we
are in the category of persons whose consent is required under Section 7 of the
Securities Act or the rules and regulations of the Commission.


                                         Very truly yours,
 
                                          /s/ Hale and Dorr LLP
                                          ---------------------
 
                                          HALE AND DORR LLP