<PAGE>   1
     As filed with the Securities and Exchange Commission on March __, 1999  

                                                    Registration No. 333-
                                                                           -----
 

                       SECURITIES AND EXCHANGE COMMISSION
                             WASHINGTON, D.C. 20549

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

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

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

                         FRONTIER FINANCIAL CORPORATION
             (Exact name of Registrant as specified in its charter)

<TABLE>
<S>                                                 <C>       
          Washington                                    91-1223535
-------------------------------                      ----------------
(State or other jurisdiction of                      (I.R.S. Employer
 incorporation or organization)                     Identification No.)
</TABLE>


                            323 S.W. Everett Mall Way
                                  P.O. Box 2215
                            Everett, Washington 98203

    (Address, including ZIP code, and telephone number, including area code,
                   of Registrant's principal executive office)

                      ------------------------------------
                          
                         FRONTIER FINANCIAL CORPORATION
                         1999 EMPLOYEE STOCK AWARD PLAN
                            (Full Title of the Plan)

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

                               James F. Felicetty
                             Secretary and Treasurer
                         Frontier Financial Corporation
                            323 S.W. Everett Mall Way
                                  P.O. Box 2215
                            Everett, Washington 98203
                                 (425) 514-0719

          (Name and address, including ZIP code, and telephone number,
                   including area code, of agent of service)

                      ------------------------------------
                                 with copies to:
                                Glen P. Garrison
                                Lucas D. Schenck
                             Keller Rohrback L.L.P.
                          1201 Third Avenue, Suite 3200
                         Seattle, Washington 98101-3052
                                 (206) 623-1900
 
                     (Name and address, including ZIP code,
                   and telephone number, including area code)

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

                         CALCULATION OF REGISTRATION FEE
<TABLE>
<CAPTION>
==============================================================================================================
<S>                             <C>                 <C>                  <C>                  <C>
                                                     Proposed Maximum     Proposed Maximum
 Title of Securities to be      Amount to be        Offering Price Per   Aggregate Offering      Amount of
        Registered              Registered(1)            Share(1)              Price          Registration Fee
--------------------------------------------------------------------------------------------------------------
Common Shares, No Par Value         10,000                $49.00              $490,000            $136.22
==============================================================================================================
</TABLE>

(1)  Estimated solely for the purpose of calculating the registration fee
pursuant to Rule 457 based upon the average of the high and low prices of the
Common Stock on March 1, 1999 as reported in The Wall Street Journal for Nasdaq
National Market Issues.

<PAGE>   2

                                     PART II

                      INFORMATION REQUIRED IN REGISTRATION


ITEM 3. INCORPORATION OF DOCUMENTS BY REFERENCE

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

     (a)  The Registrant's Annual Report on Form 10-K for the fiscal year ended
December 31, 1997, filed with the Securities and Exchange Commission (the
"Commission") on March 25, 1998 under Section 13(a) of the Securities Exchange
Act of 1934, as amended (the "Exchange Act"), which contains certified financial
statements for the most recent fiscal year for which such statements have been
filed; 

     (b)  All other reports filed by the Registrant pursuant to Section 13(a) or
15(d) of the Exchange Act since the end of the fiscal year covered by the Annual
Report on Form 10-K referred to in (a) above; and

     (c)  The description of the Registrant's Common Stock contained in the
Registrant's Prospectus/Proxy Statement and included in the Registration 
Statement on Form S-4 filed with the Commission (Registration No. 333-36929), 
including any amendments or reports filed for the purpose of updating such 
description.

     In addition, any document filed by the Registrant pursuant to Section
13(a), 13(c), 14 or 15(d) of the Exchange Act after the date hereof and prior to
the filing of a post-effective amendment that indicates that the securities
offered hereby have been sold or that deregisters the securities covered hereby
then remaining unsold shall also be deemed to be incorporated by reference into
this Registration Statement and to be a part hereof commencing on the respective
date on which such document is filed.

ITEM 4. DESCRIPTION OF SECURITIES

     Not applicable.

ITEM 5. INTERESTS OF NAMED EXPERTS AND COUNSEL

     Not applicable.

ITEM 6. INDEMNIFICATION OF DIRECTORS AND OFFICERS

     Sections 23B.08.500 through 23B.08.600 of the Washington Business
Corporation Act (the "WBCA") authorize a court to award, or a corporation's
board of directors to grant, indemnificaiton to directors, officers, employees
and agents of the Registrant and those serving at the Registrant's request in
similar positions in any other corporation, partnership, joint venture, trust or
other enterprise in terms sufficiently broad to permit such indemnification
under certain circumstances for liabilities (including reimbursement for
expenses incurred) arising under the Securities Act of 1933, as amended (the
"Securities Act"). Article IX of the Registrant's Bylaws provides for
indemnification of the Registrant's directors and officers, to the maximum
extent permitted by Washington law, against expenses and liabilities (including
any obligation to pay any judgment, settlement, fine or expenses, including
attorneys' fees) actually and reasonably incurred in connection with any
threatened, pending or completed action, suit or proceeding, whether civil,
criminal, administrative or investigative, in which the director or officer is,
was or is threatened to be made a party to such action or is otherwise involved
in such action by reason of serving or having served at the request of the
Registrant as a director or officer of another corporation.

     Section 23B.08.320 of the WBCA authorizes a corporation to limit a
director's liability to the corporation or its shareholders for monetary damages
for acts or omissions as a director, except in certain circumstances involving
intentional misconduct, self-dealing or illegal corporate loans or
distributions, or any transaction from which the director personally receives a
benefit in money, property or services to which the director is not legally
entitled. The Registrant's Articles of Incorporation provide, to the fullest
extent permitted by Washington law, limitations on a director's liability to 

<PAGE>   3

the Registrant and its shareholders. The Registrant also maintains an insurance
policy insuring its directors and officers against liability for certain acts or
omissions while acting in their official capacity.

ITEM 7. EXEMPTION FROM REGISTRATION CLAIMED

     Not applicable.

ITEM 8. EXHIBITS

     See Exhibit Index.

ITEM 9. UNDERTAKINGS

     A.   The undersigned Registrant hereby undertakes:

     1.   To file, during any period in which offers or sales are being made, a
post-effective amendment to the 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 this 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 this Registration
Statement; and

          iii. To include any material information with respect to the plan of
distribution not previously disclosed in this Registration Statement or any
material change to such information in this Registration Statement; provided,
however, that paragraphs (1)(i) and (1)(ii) above do not apply if the
information required to be included in a post-effective amendment by those
paragraphs is contained in periodic reports filed with or furnished to the
Commission by the Registrant pursuant to Section 13 or Section 15(d) of the
Exchange Act that are incorporated by reference in this Registration Statement.

     2.   That, for the purpose of determining any liability under 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.

     3.   To remove from registration by means of a post-effective amendment any
of the securities being registered that remain unsold at the termination of the
offering.

     B.   The undersigned Registrant hereby undertakes that, for purposes of
determining any liability under the Securities Act of 1933, each filing of the
Registrant's annual report pursuant to Section 13(a) or 15(d) of the Exchange
Act of 1934 (and, where applicable, each filing of any employee benefit plan's
annual report pursuant to Section 15(d) of the Exchange Act of 1934) that is
incorporated by reference in this 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 Registrant pursuant to the foregoing provisions or otherwise, the
Registrant has been advised that, in the opinion of the Securities and Exchange
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
Registrant of expenses incurred or paid by a director, officer or controlling
person of the Registrant 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 Registrant 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 of 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.

<PAGE>   4

                                   SIGNATURES

     Pursuant to the requirements of the Securities Act, 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 Everett, State of Washington, on the 1st day of March
1999.

                                        FRONTIER FINANCIAL CORPORATION



                                     By: /s/Robert J. Dickson
                                            ------------------------------------
                                            Robert J. Dickson
                                            President and Chief Executive 
                                            Officer


                                POWER OF ATTORNEY

     Each person whose signature appears below constitutes and appoints Robert
J. Dickson and James F. Felicetty, and each of them, as true and lawful
attorneys-in-fact and agents with full power of substitution and resubstitution,
to sign in the name and on behalf of such person, individually and in each
capacity stated below, any or all amendments (including pre-effective and
post-effective amendments) to this Registration Statement, and to file the same
with all exhibits thereto, and other documents in connection therewith, with the
Securities and Exchange Commission.

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

<TABLE>
<CAPTION>
SIGNATURE                           TITLE                                     DATE
---------                           -----                                     ----
<S>                                 <C>                                       <C>

By: /s/ Robert J. Dickson         President and Chief Executive Officer      March 1, 1999
    --------------------------     (Principal Executive Officer)
    ROBERT J. DICKSON


By: /s/ James F. Felicetty        Secretary and Treasurer                    March 1, 1999
    -------------------------      (Principal Financial and Accounting
    JAMES F. FELICETTY             Officer)


By: /*/                            Director                                   March 1, 1999
    -------------------------
    GEORGE E. BARBER


By: /*/                            Director                                   March 1, 1999
    -------------------------
    MICHAEL J. CORLISS


By: /*/                            Director                                   March 1, 1999
    -------------------------
    LUCY DEYOUNG


By: /*/                            Director                                   March 1, 1999
    -------------------------
    DAVID A. DUJARDIN


By: /*/                            Director                                   March 1, 1999
    -------------------------
    EDWARD D. HANSEN
</TABLE>

<PAGE>   5

<TABLE>
<S>                                <C>                                        <C>


By: /*/                            Director                                   March 1, 1999
    -------------------------
    WILLIAM H. LUCAS


By: /*/                            Director                                   March 1, 1999
    -------------------------
    JAMES M. MULLIGAN


By: /*/                            Director                                   March 1, 1999
    -------------------------
    J. DONALD REGAN


By: /*/                            Director                                   March 1, 1999
    -------------------------
    ROGER L. RICE


By: /*/                            Director                                   March 1, 1999
    -------------------------
    ROY A. ROBINSON


By: /*/                            Director                                   March 1, 1999
    -------------------------
    WILLIAM J. ROBINSON


By: /*/                            Director                                   March 1, 1999
    -------------------------
    DARREL J. STORKSON


By: /*/                            Director                                   March 1, 1999
    -------------------------
    EDWARD C. RUBATINO
</TABLE>



                                    *By: /s/Robert J. Dickson
                                            ------------------------------------
                                            Robert J. Dickson
                                            Attorney-in-Fact


     Robert J. Dickson, by signing his name hereto, does sign this document on
behalf of the persons named above, pursuant to a power of attorney duly executed
by such persons.

<PAGE>   6

                                INDEX TO EXHIBITS

<TABLE>
<CAPTION>
  EXHIBIT
   NUMBER      DESCRIPTION
  -------      -----------
<S>            <C>                                                                                                 
     5.1       Opinion of Keller Rohrback L.L.P. regarding legality of the Common Stock being 
               registered.

    23.1       Consent of Moss Adams L.L.P.

    23.2       Consent of Keller Rohrback L.L.P. (included in its Opinion filed as Exhibit 5.1)

    24.1       Power of Attorney (included on the Signature Page of this Registration
               Statement)

    99.1       Frontier Financial Corporation 1999 Employee Stock Award Plan
</TABLE>