<PAGE>   1
 
     AS FILED WITH THE SECURITIES AND EXCHANGE COMMISSION ON JUNE 22, 1998.
                                                    REGISTRATION NO. 333-
================================================================================
 
                       SECURITIES AND EXCHANGE COMMISSION
                             WASHINGTON, D.C. 20549
                            ------------------------
 
                                    FORM S-8
                             REGISTRATION STATEMENT
                                     UNDER
                           THE SECURITIES ACT OF 1933
                            ------------------------
 
                           WASHINGTON BANKING COMPANY
             (EXACT NAME OF REGISTRANT AS SPECIFIED IN ITS CHARTER)
 
<TABLE>
<S>                                            <C>
                 WASHINGTON                                     91-1725825
        (STATE OR OTHER JURISDICTION                         (I.R.S. EMPLOYER
      OF INCORPORATION OR ORGANIZATION)                   IDENTIFICATION NUMBER)
</TABLE>
 
             1421 S.W. BARLOW STREET, OAK HARBOR, WASHINGTON 98277
   (ADDRESS, INCLUDING ZIP CODE, AND TELEPHONE NUMBER, INCLUDING AREA CODE OF
                   REGISTRANT'S PRINCIPAL EXECUTIVE OFFICES)
 
                        WHIDBEY ISLAND BANK 401(K) PLAN
                            (FULL TITLE OF THE PLAN)
 
                                 MICHAL D. CANN
                     PRESIDENT AND CHIEF EXECUTIVE OFFICER
                           WASHINGTON BANKING COMPANY
                            1421 S.W. BARLOW STREET
                          OAK HARBOR, WASHINGTON 98277
                                 (360) 679-3121
            (NAME, ADDRESS, INCLUDING ZIP CODE AND TELEPHONE NUMBER,
                   INCLUDING AREA CODE, OF AGENT FOR SERVICE)
 
                          COPIES OF COMMUNICATIONS TO:
 
                           SANDRA L. GALLAGHER, ESQ.
        GORDON, THOMAS, HONEYWELL, MALANCA, PETERSON & DAHEIM, P.L.L.C.
                             2200 WELLS FARGO PLAZA
                            TACOMA, WASHINGTON 98402
                                 (206) 572-5050
 
                        CALCULATION OF REGISTRATION FEE
 
<TABLE>
<S>                        <C>                   <C>                   <C>                   <C>
=================================================================================================================
                                                                         PROPOSED MAXIMUM
  TITLE OF SECURITIES         AMOUNT TO BE        MAXIMUM OFFERING     AGGREGATE OFFERING        AMOUNT OF
    TO BE REGISTERED          REGISTERED(1)       PRICE PER SHARE(2)         PRICE(2)          REGISTRATION FEE
-----------------------------------------------------------------------------------------------------------------
Common Stock no par value        200,000                $13.00              $2,600,000             $767.00
=================================================================================================================
</TABLE>
 
(1) Represents an estimate of such presently undeterminable number of shares of
    Washington Banking Company, parent company of Whidbey Island Bank, as may be
    purchased with employee contributions pursuant to the Whidbey Island Bank
    401(k) Plan (the "Plan"). In addition, pursuant to Rule 416(c) under the
    Securities Act of 1933 (the "Act"), this registration statement also covers
    an indeterminate amount of interests to be offered or sold pursuant to the
    Plan.
 
(2) Estimated solely for the purpose of calculating the registration fee
    pursuant to Rule 457(h).
================================================================================
<PAGE>   2
 
                                     PART I
 
              INFORMATION REQUIRED IN THE SECTION 10(A) PROSPECTUS
 
     The documents containing the information required by Part I of this
registration statement on Form S-8 (the "Registration Statement") will be sent
or given to Plan participants as specified by Rule 428(b)(1) under the Act. Such
documents and the documents incorporated by reference herein pursuant to Item 3
of Part II hereof, taken together, constitute a prospectus that meets the
requirements of Section 10(a) of the Act.
 
                                    PART II
 
               INFORMATION REQUIRED IN THE REGISTRATION STATEMENT
 
ITEM 3. INCORPORATION OF DOCUMENTS BY REFERENCE
 
     The following documents have been filed with the Commission by Washington
Banking Company (the "Company") and are incorporated herein by reference and
made a part hereof:
 
          1. The Company's Prospectus filed with the Commission on April 10,
     1998 as part of the Company's Registration Statement on Form SB-2 filed
     under Registration Number 333-49925, as amended on June 3, 1998 and
     thereafter, containing audited financial statements for the years ending
     December 31, 1997, 1996 and 1995 and unaudited financial statements dated
     as of March 31, 1998.
 
          2. The description of the Common Stock of the Company contained in the
     Company's Registration Statement on Form 8-A filed with the Commission.
 
          3. All documents filed by the Company and the Plan pursuant to
     Sections 13(a), 13(c), 14 and 15(d) of the Securities Exchange Act of 1934
     subsequent to the date hereof and prior to the filing of a post-effective
     amendment which indicates that all securities offered have been sold or
     which deregisters all securities then remaining unsold shall be deemed to
     be incorporated by reference herein and to be part hereof from the date of
     the 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
 
     The Washington Business Corporation Act, RCW Chapter 23B.08, authorizes
indemnification of directors, officers and employees under certain
circumstances. The Company's Articles of Incorporation provide, among other
things, for the indemnification of the Company's directors and those of any
subsidiary of the Company, and authorize the Board to pay reasonable expenses
incurred by, or to satisfy a judgment or fine against, a current or former
director in connection with any personal legal liability incurred by the
individual by reason of the fact that that individual is or was a director and
which was not the result of conduct finally adjudged to be "egregious" conduct.
"Egregious" conduct is defined as intentional misconduct, a knowing violation of
law, or participation in any transaction from which the person will personally
receive a benefit in money, property or services to which that person is not
legally entitled. The Articles of Incorporation also include a provision that
limits the liability of the Company's directors and those of any subsidiary from
any personal liability to the Company, such subsidiary or their respective
shareholders for conduct not found to have been egregious. The Company has
purchased an officers and directors liability insurance policy which provides
for insurance of directors and officers of the Company against certain
liabilities they may incur in their capacities as such.
 
                                        2
<PAGE>   3
 
ITEM 7. EXEMPTION FROM REGISTRATION CLAIMED
 
     Not Applicable
 
ITEM 8. EXHIBITS
 
<TABLE>
<CAPTION>
EXHIBIT NO.                             EXHIBITS
-----------                             --------
<C>           <S>
      4       Articles of Incorporation filed as an exhibit to the
              Company's Registration Statement on Form SB-2 (Registration
              No. 333-49925) and incorporated by this reference.
   23.1       Consent of KPMG Peat Marwick LLP
   23.2       Consent of David Christensen CPA & Consultant PLLC.
     24       Power of Attorney pursuant to which the Directors have
              signed this Form S-8 Registration Statement
</TABLE>
 
---------------
 
 *  Filed herewith.
 
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:
 
             (i) To include any prospectus required by Section 10(a)(3) of the
        Securities Act of 1933;
 
             (ii) To reflect in the prospectus any facts or events arising after
        the effective date of the 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 the registration statement;
 
             (iii) 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;
 
             Provided, however, that paragraph (a)(1)(i) and (a)(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 by
        the Company pursuant to section 13 or section 15(d) of the Securities
        Exchange Act of 1934 that are incorporated by reference in this
        registration statement.
 
          (2) That, for the purpose of determining any liability under the
     Securities Act of 1933, 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.
 
          (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 undersigned Company hereby undertakes that, for purposes of
determining any liability under the Securities Act of 1933, each filing of the
Company's annual report pursuant to Section 13(a) or Section 15(d) of the
Securities Exchange Act of 1934 (and, where applicable, each filing of an
employee benefit plan's annual report pursuant to Section 15(d) of the
Securities Exchange Act of 1934) that is incorporated by reference in the
registration statement shall be deemed to be a new registration statement
relating to the
 
                                        3
<PAGE>   4
 
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 of 1933 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 Securities and Exchange Commission such
indemnification is against public policy as expressed in the 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 persons of the Company in the
successful defense of any action, suit or proceeding) is asserted by such
director, officer or controlling persons in connection with the securities being
registered, the Company 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 Act and will be governed by the final adjudication of
such issue.
 
     (d) The Company hereby undertakes that the registrant will submit the Plan
and any amendment thereto to the Internal Revenue Service ("IRS") in a timely
manner, and will make all changes required by the IRS in order to qualify the
Plan.
 
                                        4
<PAGE>   5
 
                                   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 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 Oak Harbor, State of Washington, on June 22, 1998.
 
                                          WASHINGTON BANKING COMPANY
                                          (Registrant)
 
                                          By:      /s/ MICHAL D. CANN
                                            ------------------------------------
                                                       Michal D. Cann
                                               President and Chief Executive
                                                           Officer
 
     Pursuant to the requirements of the Securities Act of 1933, this
Registration Statement has been signed by the following persons in the
capacities indicated, on June 22, 1998.
 
<TABLE>
<CAPTION>
                      SIGNATURE                                             TITLE
                      ---------                                             -----
<S>                                                     <C>
 
                 /s/ MICHAL D. CANN                         President and Chief Executive Officer
-----------------------------------------------------
                   Michal D. Cann
 
                 /s/ LARRY SCODELLER                               Executive Vice President
-----------------------------------------------------            and Chief Operating Officer
                   Larry Scodeller
 
                   Michal D. Cann                                          Director
 
                     Orlan Dean                                            Director
 
                   Marlen Knutson                                          Director
 
                 Karl C. Krieg, III                                        Director
 
                     Jay T. Lien                                           Director
 
                   Robert B. Olson                                         Director
 
                Anthony B. Pickering                                       Director
 
                  Alvin J. Sherman                                         Director
 
               Edward J. (Bud) Walgren                                     Director
</TABLE>
 
     Michal D. Cann, by signing his name hereto, does hereby sign this document
in his capacity as a director and pursuant to powers of attorney duly executed
by the persons named, filed with the Securities and Exchange Commission as an
exhibit to this document, on behalf of such persons, all in the capacities and
on the date stated, such persons including a majority of the directors of the
registrant.
 
                                                  /s/ MICHAL D. CANN
 
                                          --------------------------------------
                                                      Michal D. Cann
                                                     Attorney-in-Fact
 
                                        5
<PAGE>   6
 
     Pursuant to the requirements of the Securities Act of 1933, the trustee of
the Plan has duly caused this Registration Statement to be signed on its behalf
by the undersigned, thereunto duly authorized, in the City of Oak Harbor, State
of Washington, on June 22, 1998.
 
                                          WHIDBEY ISLAND BANK 401(K) PLAN
                                          (Plan)
 
                                          By:     /s/ MICHAL D. CANN
                                          --------------------------------------
                                                      Michal D. Cann
                                                         Trustee
 
                                          By:    /s/ EDWARD J. WALGREN
                                          --------------------------------------
                                                 Edward J. (Bud) Walgren
                                                         Trustee
 
                                        6
<PAGE>   7
 
                                 EXHIBIT INDEX
 
<TABLE>
<CAPTION>
                                                                              SEQUENTIALLY
                                                                                NUMBERED
EXHIBIT NO.                             EXHIBITS                                  PAGE
-----------                             --------                              ------------
<C>           <S>                                                             <C>
      4       Articles of Incorporation filed as an exhibit to the
              Company's Registration Statement on Form SB-2 (Registration
              No. 333-49925) and incorporated by this reference...........
   23.1       Consent of KPMG Peat Marwick LLP............................
   23.2       Consent of David Christensen CPA & Consultant PLLC..........
     24       Power of Attorney pursuant to which the Directors have
              signed this Form S-8 Registration Statement.................
</TABLE>
 
---------------
 
 *  Filed herewith.
 
                                        7