<PAGE>   1

                                  UNITED STATES
                       SECURITIES AND EXCHANGE COMMISSION
                             Washington, D.C. 20549
                                    Form 10-K

(X)      Annual report pursuant to Section 13 or 15(d) of the Securities
         Exchange Act of 1934 (Fee Required) For the fiscal year ended December
         31, 1998.
                                       OR

( )      Transition report pursuant to Section 13 or 15(d) of the Securities
         Exchange Act of 1934 (No Fee Required)

         For the transition period from              to               .
                                        -------------    --------------

                           Commission File No. 0-22124
                                               -------

                                NSD Bancorp, Inc.
                                -----------------
             (Exact name of Registrant as specified in its charter)

Commonwealth of Pennsylvania                           25-1616814
(State or other jurisdiction of             (I.R.S. Employer Identification No.)
incorporation or organization)

               5004 McKnight Road, Pittsburgh, Pennsylvania 15237
               (Address of principal executive offices) (Zip Code)

                                 (412) 231-6900
              (Registrant's telephone number, including area code)

<TABLE>
<CAPTION>
<S>                                                                    <C>              
Securities registered pursuant to Section 12(b) of the Act:
         Title of each class                                           Name of each exchange on which registered

         -------------------                                              -------------------
</TABLE>

Securities registered pursuant to Section 12(g) of the Act:

                          Common Stock, $1.00 Par Value
                          -----------------------------

Indicate by check mark whether the Registrant (1) has filed all reports required
to be filed by Section 13 or 15(d) of the Securities Exchange Act of 1934 during
the preceding 12 months (or for such shorter period that the registrant was
required to file such reports), and (2) has been subject to such filing
requirements for the past 90 days.

                                    YES X NO
                                       ---  ---

Indicate by check mark if disclosure of delinquent filers pursuant to Item 405
of Regulation S-K (ss.229.405 of this chapter) is not contained herein, and will
not be contained, to the best of the registrant's knowledge, in definitive proxy
or information statements incorporated by reference in Part III of this Form
10-K or any amendment to this Form 10-K
                                        ----------

The aggregate market value of Common Stock, $1.00 par value, held by
non-affiliates of March 2, 1999, was $65,336,330.

The number of shares outstanding of the Registrant's Common Stock as of March 2,
1999 was 2,840,710.

Documents Incorporated By Reference:
Portions of the Registrant's Annual Report to Shareholders for the year ended
December 31, 1998 are incorporated by reference into Parts I, II and IV.

Portions of the Registrant's Proxy Statement for the Annual Shareholders'
Meeting to be held on April 27, 1999, are incorporated by reference into Part
III.

                      Number of Pages in this Filing  6
                                                     ----



<PAGE>   2



                               NSD BANCORP, INC.
                                   FORM 10-K
                  For the Fiscal Year Ended December 31, 1998

                                     INDEX

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                                                                                         Page
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<S>               <C>                                                                   <C>
Part I

         Item 1.  Business                                                                1

         Item 2.  Properties                                                              2

         Item 3.  Legal Proceedings                                                       3

         Item 4.  Submission of Matters to a Vote of Security Holders                     3

Part II

         Item 5.  Market for Registrant's Common Equity and Related
                  Stockholder Matters                                                     3

         Item 6.  Selected Financial Data                                                 3

         Item 7.  Management's Discussion and Analysis of Financial condition
                  and Results of Operations                                               3

         Item 7a. Quantitative and Qualitative Disclosure About Market Risk               3

         Item 8.  Financial Statements and Supplementary Data                             3

         Item 9.  Changes in and Disagreements With Accountants on Accounting
                  and Financial Disclosure                                                3

Part III

         Item 10. Directors and Executive Officers of the Registrant                      4

         Item 11. Executive Compensation                                                  4

         Item 12. Security Ownership of Certain Beneficial Owners and
                  Management                                                              4

         Item 13. Certain Relationships and Related Transactions                          4

Part IV

         Item 14. Exhibits, Financial Statement Schedules and Reports on
                  Form 8-K                                                                4

                  Signatures                                                              6
</TABLE>



<PAGE>   3



Part I

ITEM 1.  BUSINESS

NSD Bancorp, Inc. (the "Corporation") is a registered bank holding company
organized under the Pennsylvania Business Corporation Law and is registered
under the Bank Holding Company Act of 1956, as amended. The Corporation became a
holding company upon acquiring all of the outstanding shares of NorthSide Bank
through an exchange of stock on August 2, 1993. At December 31, 1998, the
Corporation had total assets, deposits and shareholders' equity of $357,167,992,
$280,115,160 and $32,394,365, respectively. Full-time equivalent employees of
the Corporation were 137 at December 31, 1998.

The Corporation derives substantially all of it's income from banking and
bank-related services provided by its wholly-owned subsidiary, NorthSide Bank
(the Bank). The Bank is a state chartered bank with ten branch locations at
December 31, 1998. The Corporation is subject to periodic examination and
regulation by the Federal Reserve Bank. As a state chartered bank, NorthSide
Bank is subject to periodic examination and regulation by the Pennsylvania
Department of Banking and the FDIC. The Bank is a full-service bank offering
retail banking services, such as demand, savings and time deposits, money market
accounts, secured and unsecured loans, mortgage loans, safe deposit boxes,
holiday club accounts, wire transfers, money orders and traveler's checks.
Services to commercial customers are also offered, including real estate
mortgage loans, lines of credit, inventory and accounts receivable financing and
equipment leasing. NorthSide Bank operates nine automatic teller machines to
provide 24 hour banking services to its customers. The Bank's deposits are
derived from more than 50,000 individual and commercial accounts. There is no
single depositor or group of related depositors, the loss of whom would have a
materially adverse effect on the business of the Bank. The Bank's loans are not
concentrated within a single industry or group of related industries to any
material extent.

The Bank's service area includes the northern portion of Allegheny County and
southern Butler County where it competes with many other banks. The Bank also
competes with regional bank and trust companies, credit unions, savings and loan
associations, consumer finance companies, insurance companies and direct lending
agencies of the government throughout its service area. Banks compete for all
types of deposit and loan accounts, with banks and trust companies having the
additional power to compete for trust accounts. Savings and loan associations
offer savings and time deposit services as well as installment and mortgage
loans.

Credit unions also compete with the Bank for savings and time deposit accounts
and for installment loan accounts. Consumer finance companies provide personal
installment loan services in direct competition with the Bank.

The Bank's business is not seasonal in nature, nor does it depend on any single
customer or a few customers, the loss of any one or more of which would be a
materially adverse effect on its business. A further description of the
Corporation's business and discussion of operations is set forth on page 8 in
the Corporation's 1998 Annual Report to Shareholders included in this Form 10-K
as Exhibit 13 which description is incorporated herein by reference.


                                       1
<PAGE>   4



ITEM 2.  PROPERTIES

The Corporation's principal office is located at 5004 McKnight Road, Pittsburgh,
Pennsylvania. The Bank's main office is located at 100 Federal Street,
Pittsburgh, Pennsylvania. Including the main office, the Bank has a total of ten
branch offices located as listed below and on the following page. The Bank owns
the four-story building located on Federal Street, and a parking lot adjacent to
the building. The Bank also owns its Cranberry Township, Ross Township, Pine
Creek and West View branch offices, although it leases the land on which the
Pine Creek Shopping Center branch office is located. The Bank's Hampton
Township, McCandless Township, Allegheny Professional Building, Duncan Manor and
Pittsburgh Cultural District branch offices and the Pine Creek Shopping Center
land are operated under leases that contain various renewal option periods
extending through September, 2005.

                            Location Name and Address
                            -------------------------
                              Pittsburgh-North Side
                               100 Federal Street
                              Pittsburgh, PA 15212

                         Allegheny Professional Building
                              490 East North Avenue
                              Pittsburgh, PA 15212

                                    West View
                                728 Center Avenue
                               West View, PA 15229

                               McCandless Township
                         Perry Highway and Ingomar Road
                              McCandless, PA 15237

                               Cranberry Township
                          Route 19N at St. Francis Way
                               Cranberry, PA 16066

                                Hampton Township
                            Shoppers Plaza at Route 8
                                Hampton, PA 15101

                                  Ross Township
                               5004 McKnight Road
                                 Ross, PA 15237

                                   Pine Creek
                           Pine Creek Shopping Center
                               9805 McKnight Road
                              McCandless, PA 15237

                                  Duncan Manor
                                  Duncan Plaza
                               1701 Duncan Avenue
                             Allison Park, PA 15101

                          Pittsburgh Cultural District
                               701 Liberty Avenue
                              Pittsburgh, PA 15222


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ITEM 3.  LEGAL PROCEEDINGS

The Corporation is subject to a number of asserted and unasserted potential
legal claims encountered in the normal course of business. In the opinion of
both management and legal counsel, there is no present basis to conclude that
the resolution of these claims will have a material adverse effect on the
Corporation's consolidated financial position or results of operations.

ITEM 4.  SUBMISSION OF MATTERS TO A VOTE OF SECURITY HOLDERS

No matter was submitted by the Corporation to its shareholders through the
solicitation of proxies or otherwise during the fourth quarter of the fiscal
year covered by this report.

PART II

ITEM 5.  MARKET FOR REGISTRANT'S COMMON EQUITY AND RELATED STOCKHOLDER MATTERS

The Corporation's Common Stock is traded on the NASDAQ National Market System
under the symbol NSDB. The information set forth under the captions "Regulatory
Restrictions" and "Sales Price and Cash Dividends per Share" on pages 19 and 37,
respectively, of the 1998 Annual Report is incorporated herein by reference. As
of March 16, 1999, the Corporation had 437 shareholders of its Common Stock.

ITEM 6.  SELECTED FINANCIAL DATA

Incorporated by reference is the information presented on pages 25 and 28 of the
1998 Annual Report.

ITEM 7.  MANAGEMENT'S DISCUSSION AND ANALYSIS OF FINANCIAL CONDITION AND
         RESULTS OF OPERATIONS

Incorporated by reference is the information presented on pages 27 to 39 of the
1998 Annual Report.

ITEM 7A. QUANTITATIVE AND QUALITATIVE DISCLOSURE ABOUT MARKET RISK

Incorporated by reference to the information presented on pages 37 to 38 of the
1998 Annual Report attached as Exhibit 13 hereto.

ITEM 8.  FINANCIAL STATEMENTS AND SUPPLEMENTARY DATA

Incorporated by reference is the information presented on pages 4 to 26 of the
1998 Annual Report.

ITEM 9.  CHANGES IN AND DISAGREEMENTS WITH ACCOUNTANTS ON ACCOUNTING AND 
         FINANCIAL DISCLOSURE

None.

         
                                       3
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PART III

ITEM 10. DIRECTORS AND EXECUTIVE OFFICERS OF THE REGISTRANT

Incorporated by reference is the information presented on pages 3,4 and 5 of the
Proxy Statement for the Annual Shareholders Meeting to be held April 27, 1999.

ITEM 11. EXECUTIVE COMPENSATION

Incorporated by reference is the information presented on pages 5, 6, 7, and 8
of the Proxy Statement for the Annual Shareholders Meeting to be held on April
27, 1999.

ITEM 12. SECURITY OWNERSHIP OF CERTAIN BENEFICIAL OWNERS AND MANAGEMENT

Incorporated by reference is the information presented on pages 2, 8 and 9 of
the Proxy Statement for the Annual Shareholders Meeting to be held on April 27,
1999.

ITEM 13. CERTAIN RELATIONSHIPS AND RELATED TRANSACTIONS

Incorporated by reference is the information presented on pages 8 and 9 of the
Proxy Statement for the Annual Shareholders Meeting to be held on April 27,
1999.

PART IV

ITEM 14. EXHIBITS, FINANCIAL STATEMENT SCHEDULES AND REPORTS ON FORM 8-K

CONSOLIDATED FINANCIAL STATEMENTS FILED:
The consolidated financial statements and report of the Registrant's independent
accountant thereon are incorporated by reference to the pages indicated in the
said Annual Report.

CONSOLIDATED FINANCIAL STATEMENTS:
NSD Bancorp, Inc. and Subsidiary
         Consolidated Balance Sheet,  page 4
         Consolidated Statement of Income,  page 5
         Consolidated Statement of Cash Flows,  page 6
         Consolidated Statement of Changes in Shareholders' Equity,  page 7
         Notes to Consolidated Financial Statements,  pages 8-25
         Independent Auditor's Report,  page 26

CONSOLIDATED FINANCIAL STATEMENT SCHEDULES:
Schedules normally required on Form 10-K are omitted since the required
information is either not applicable, not deemed material or is shown in the
respective consolidated financial statements or in the notes thereto.





                                       4
<PAGE>   7




EXHIBITS

         3a.      Articles of Incorporation of NSD Bancorp, Inc. filed as
                  Exhibit 3a to NSD Bancorp, Inc.'s Form S-4 filed March 9, 1993
                  (Registration No. 33-59242), is incorporated herein by
                  reference.

         3b.      Bylaws of NSD Bancorp, Inc. filed as Exhibit 3b to NSD
                  Bancorp, Inc.'s Form S-4 filed March 9, 1993 (Registration No.
                  33-59242), is incorporated herein by reference.

         10A.     Employment agreement, dated July 1, 1993 between NSD Bancorp,
                  Inc. And Lloyd Gibson filed as exhibit 10D to NSD Bancorp,
                  Inc.'s Form 10-K for the fiscal year ended December 31, 1993
                  is incorporated herein by reference.

         10B.     NSD Bancorp, Inc. 1994 Stock Option Plan filed as Exhibit 4.1
                  to NSD Bancorp, Inc.'s Form S-8 filed April 27, 1994 is
                  incorporated herein by reference.

         10C.     NSD Bancorp, Inc. 1994 Non-Employee Director Stock Option Plan
                  filed as Exhibit 4.1 to NSD Bancorp, Inc.'s Form S-8 filed
                  April 27, 1994 is incorporated herein by reference.

         11.      Statement re: Computation of Earnings Per Share (incorporated
                  by reference to p. 10 of 1998 Annual Report, attached as
                  Exhibit 13, hereto.)

         13.      1998 Annual Report to Shareholders is presented within.

         21.      Subsidiaries of the Registrant are presented within.

         23.1     Consent of Deloitte & Touche LLP, Independent Certified 
                  Public Accountants is presented within.

         23.2     Consent of PricewaterhouseCoopers LLP, Independent Certified 
                  Public Accountants is presented within.     

REPORTS ON FORM 8-K

       None.









                                       5
<PAGE>   8

                                   SIGNATURES

Pursuant to the requirements of Section 13 or 15(d) of the Securities Exchange
Act of 1934, the registrant has duly caused this report to be signed on its
behalf by the undersigned, thereunto duly authorized.

                                                         NSD BANCORP, INC.
                                                             (Registrant)

Dated:   March 26, 1999                         By   /S/  Lloyd G. Gibson       
                                                     ---------------------------
                                                     Lloyd G. Gibson
                                                     President and Chief
                                                     Executive Officer

Pursuant to the requirements of the Securities Exchange Act of 1934, this report
has been signed below by the following persons on behalf of the registrant and
in the capacities indicated on March 26, 1999:


<TABLE>
<CAPTION>
<S>                                 <C>
/S/ Lawrence R. Gaus                Lawrence R. Gaus, Chairman of the Board, Director
---------------------------------

/S/ Lloyd G. Gibson                 Lloyd G. Gibson, President and Chief Executive
---------------------------------   Officer, Director             
                                    (Principal Executive Officer) 
                                    

/S/ James P. Radick                 James P. Radick, Treasurer
---------------------------------   (Principal Financial and Accounting Officer)
                                    

/S/ William R. Baierl               William R. Baierl, Director
---------------------------------

/S/ Grant A. Colton Jr.             Grant A. Colton, Jr., Director
---------------------------------

/S/ Nicholas C. Geanopulos          Nicholas C. Geanopulos, Director
---------------------------------

/S/ Charles S. Lenzner              Charles S. Lenzner, Director
---------------------------------

/S/ Kenneth L. Rall                 Kenneth L. Rall, Director
---------------------------------

/S/ Arthur J. Rooney II             Arthur J. Rooney, II, Director
---------------------------------
</TABLE>


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