<PAGE>

       As filed with the Securities and Exchange Commission on May 5, 2006

                                                    Registration No. 333-_______

================================================================================

                                  UNITED STATES
                       SECURITIES AND EXCHANGE COMMISSION
                             Washington, D.C. 20549

                                    FORM S-8

                             REGISTRATION STATEMENT
                                      UNDER
                           THE SECURITIES ACT OF 1933

                               UNIONBANCORP, INC.
             (Exact name of registrant as specified in its charter)

<TABLE>
<S>                                                          <C>
            DELAWARE                                              36-3145350
(State or other jurisdiction of                                (I.R.S. employer
 incorporation or organization)                              identification no.)
</TABLE>

                             122 West Madison Street
                             Ottawa, Illinois 61350
          (Address of principal executive offices, including zip code)

                               UNIONBANCORP, INC.
                                   401(K) PLAN
                            (Full title of the plan)

                                          WITH A COPY TO:

SCOTT A. YEOMAN                           TIMOTHY E. KRAEPEL
PRESIDENT AND CHIEF EXECUTIVE OFFICER     HOWARD & HOWARD ATTORNEYS, P.C.
UNIONBANCORP, INC.                        THE PINEHURST OFFICE CENTER, SUITE 101
122 WEST MADISON STREET                   39400 NORTH WOODWARD AVENUE
OTTAWA, ILLINOIS 61350                    BLOOMFIELD HILLS, MICHIGAN 48304-5151
(Name and address of agent for service)   (248) 645-1483

(815) 431-2720
(Telephone number, including area code,
of agent for service)

                     CALCULATION OF REGISTRATION FEE

<TABLE>
<CAPTION>
===================================================================================================
                                                        Proposed         Proposed
                                                         maximum         maximum
                                          Amount        offering        aggregate        Amount of
                                          to be         price per        offering      Registration
Title of Securities to be Registered    registered   share (1), (2)   price (1), (2)        fee
------------------------------------    ----------   --------------   --------------   ------------
<S>                                     <C>          <C>              <C>              <C>
Common Stock, $1.00 par value (2), (3)     27,525        $20.755         $571,282         $61.13
===================================================================================================
</TABLE>

(1)  Estimated solely for purposes of calculating the registration fee in
     accordance with Rule 457(c) and (h).

(2)  In addition, pursuant to Rule 416, this registration statement covers an
     indeterminate amount of interests to be offered or sold pursuant to the
     employee benefit plan described herein; plus, an indeterminate number of
     additional shares as may be issuable in the event of an adjustment as a
     result of an increase in the number of issued shares of Registrant's Common
     Stock resulting from a subdivision of such shares, the payment of stock
     dividends or certain other capital adjustments.

(3)  Also includes an equal number of Rights to purchase shares of Registrant's
     Series C Junior Participating Preferred Stock, which rights are not (a)
     separable from the shares of Common Stock; or (b) presently exercisable.

================================================================================
<PAGE>

                                     PART I.

     This Registration Statement on Form S-8 is filed to register 27,525
additional shares of Common Stock issuable under the UnionBancorp, Inc. 401(k)
Plan (the "Plan"). UnionBancorp, Inc.'s registration statement on Form S-8 filed
with the Securities and Exchange Commission on April 9, 1998 (File No.
333-49729) registered 20,000 shares issuable under the Plan and is incorporated
herein by reference, except to the extent that the items in this registration
statement update such information contained in the prior registration statement.
This Registration Statement is filed to register an additional 27,525 shares and
is filed pursuant to General Instruction E to Form S-8.

ITEM 1. PLAN INFORMATION*

ITEM 2. REGISTRANT INFORMATION AND EMPLOYEE PLAN ANNUAL INFORMATION*

*    Information required by Part I to be contained in this Section 10(a)
     prospectus is omitted from the Registration Statement in accordance with
     Rule 428 under the Securities Act of 1933, as amended ("Securities Act"),
     and the Note to Part I on Form S-8.

                                    PART II.

               INFORMATION REQUIRED IN THE REGISTRATION STATEMENT

ITEM 3. INCORPORATION OF DOCUMENTS BY REFERENCE

     The following documents filed with the Securities and Exchange Commission
(the "Commission") by the Registrant (File No. 0-28846) are incorporated in this
Registration Statement by reference:

     (1) the Registrant's Annual Report on Form 10-K for the year ended December
31, 2005;

     (2) all other reports filed by the Registrant pursuant to Section 13(a) or
15(d) of the Securities Exchange Act of 1934 (the "Exchange Act") since December
31, 2005;

     (3) the information contained in the Registrant's definitive Proxy
Statement filed with the Commission on March 24, 2006 relating to its 2006
Annual Meeting of Stockholders (with the exception of the compensation committee
report, the audit committee report, and the performance graph which are not
incorporated by reference); and

     (5) the description of Registrant's Common Stock, $1.00 par value which is
contained in Item 1 of Registrant's Registration Statement on Form 8-A (File No.
0-28846) filed with the Commission on September 4, 1996, including any
amendments or reports filed for the purpose of updating such description.


                                        2

<PAGE>

     All documents filed by the Registrant pursuant to Sections 13(a), 13(c), 14
and 15(d) of the Exchange Act after the date of this Registration Statement 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 in this
Registration Statement and to be a part hereof from the date of filing 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

     Section 145 of the General Corporation Law of the State of Delaware
("Section 145"), Article VIII of the Registrant's Restated Certificate of
Incorporation, as amended, and Article VII of the Registrant's By-Laws provide
for the indemnification of the Registrant's directors and officers in a variety
of circumstances, which may include indemnification for liabilities under the
Securities Act of 1933. The general effect of these provisions is to provide
that the Registrant shall indemnify its directors and officers against expenses
(including attorneys' fees), judgments, fines and amounts paid in settlement
actually and reasonably incurred by them in the proceeding in which such persons
are made a party by reason of being or having been a director or officer of
Registrant, but only if it is determined that they acted in good faith and in a
manner they reasonably believed to be in or not opposed to the best interest of
the Registrant and, with respect to any criminal action or proceeding, had no
reasonable cause to believe their conduct was unlawful. Such determination may
be made by (i) a majority vote of a quorum consisting of disinterested
directors, (ii) an independent legal counsel in a written opinion (if no such
quorum is available or if a quorum of disinterested directors so directs), or
(iii) the stockholders. The Registrant will pay expenses incurred by its
directors and officers prior to the final disposition of such action, suit or
proceeding, if the director or officer undertakes to repay such amount unless it
is ultimately determined that they are entitled to be indemnified by the
Registrant.

     The Registrant has insurance which, subject to certain policy limits,
deductible amounts and exclusions, insures directors and officers of the
Registrant for liabilities incurred as a result of acts committed in their
capacity as directors and officers or claims made against them by reason of
their status as directors or officers.

     As permitted pursuant to Section 102(b)(7) of the General Corporation Law
of the State of Delaware, the Restated Certificate of Incorporation, as amended,
of the Registrant eliminates the personal monetary liability of a director of
the Registrant or its stockholders for breach of fiduciary duty as a director,
except for liability that results from (i) any breach of the director's duty of
loyalty to the Registrant or its stockholders; (ii) acts or omissions not in
good faith or which involve intentional misconduct or a knowing violation of
law, (iii) Section 174 of the General Corporation Law of the State of Delaware
(which Section pertains to a director's


                                        3

<PAGE>

liability for unlawful payments of dividends or unlawful stock purchases or
redemptions); or (iv) any transaction from which the director derived an
improper personal benefit.

ITEM 7. EXEMPTION FROM REGISTRATION CLAIMED

     Not applicable.

ITEM 8. EXHIBITS

     The exhibits filed herewith or incorporated by reference herein are set
forth in the Exhibit index filed as part of this Registration Statement on page
8 hereof.

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 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
          paragraphs (a)(1)(i) and (a)(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 Securities Exchange Act of 1934 that are
          incorporated by reference in the 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

     (4)  That, for the purpose of determining liability under the Securities
          Act of 1933 to any purchaser:


                                        4

<PAGE>

               (a) If the registrant is relying on Rule 430B of the Securities
          Act: (i) each prospectus filed by the registrant pursuant to Rule
          424(b)(3) shall be deemed to be part of the registration statement as
          of the date the filed prospectus was deemed part of and included in
          the registration statement; and (ii) each prospectus required to be
          filed pursuant to Rule 424(b)(2), (b)(5), or (b)(7) as part of a
          registration statement in reliance on Rule 430B relating to an
          offering made pursuant to Rule 415(a)(1)(i), (vii), or (x) for the
          purpose of providing the information required by section 10(a) of the
          Securities Act of 1933 shall be deemed to be part of and included in
          the registration statement as of the earlier of the date such form of
          prospectus is first used after effectiveness or the date of the first
          contract of sale of securities in the offering described in the
          prospectus. As provided in Rule 430B, for liability purposes of the
          issuer and any person that is at that date an underwriter, such date
          shall be deemed to be a new effective date of the registration
          statement relating to the securities in the registration statement to
          which that prospectus relates, and the offering of such securities at
          that time shall be deemed to be the initial bona fide offering
          thereof. Provided, however, that no statement made in a registration
          statement or prospectus that is part of the registration statement or
          made in a document incorporated or deemed incorporated by reference
          into the registration statement or prospectus that is part of the
          registration statement will, as to a purchaser with a time of contract
          of sale prior to such effective date, supersede or modify any
          statement that was made in the registration statement or prospectus
          that was part of the registration statement or made in any such
          document immediately prior to such effective date; or

               (b) If the registrant is subject to Rule 430C, each prospectus
          filed pursuant to Rule 424(b) as part of a registration statement
          relating to an offering, other than registration statements relying on
          Rule 430B or other than prospectuses filed in reliance on Rule 430A,
          shall be deemed to be part of and included in the registration
          statement as of the date it is first used after effectiveness.
          Provided, however, that no statement made in a registration statement
          or prospectus that is part of the registration statement or made in a
          document incorporated or deemed incorporated by reference into the
          registration statement or prospectus that is part of the registration
          statement will, as to a purchaser with a time of contract of sale
          prior to such first use, supersede or modify any statement that was
          made in the registration statement or prospectus that was part of the
          registration statement or made in any such document immediately prior
          to such date of first use.

     (5)  That, for the purpose of determining liability of the registrant under
          the Securities Act of 1933 to any purchaser in the initial
          distribution of the securities: The undersigned registrant undertakes
          that in a primary offering of securities of the undersigned registrant
          pursuant to this registration statement, regardless of the
          underwriting method used to sell the securities to the purchaser, if
          the securities are offered or sold to such purchaser by means of any
          of the following communications, the undersigned registrant will be a
          seller to the purchaser and will be considered to offer or sell such
          securities to such purchaser:


                                        5

<PAGE>

          (i) Any preliminary prospectus or prospectus of the undersigned
          registrant relating to the offering required to be filed pursuant to
          Rule 424; (ii) Any free writing prospectus relating to the offering
          prepared by or on behalf of the undersigned registrant or used or
          referred to by the undersigned registrant; (iii) The portion of any
          other free writing prospectus relating to the offering containing
          material information about the undersigned registrant or its
          securities provided by or on behalf of the undersigned registrant; and
          (iv) Any other communication that is an offer in the offering made by
          the undersigned registrant to the purchaser.

(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 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 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
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 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 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.


                                        6

<PAGE>

                                   SIGNATURES

     The Registrant. Pursuant to the requirements of the Securities Act of 1933,
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 Ottawa, State of Illinois, on May 5, 2006.

                                        UNIONBANCORP, INC.
                                        (Registrant)


                                        By: /s/ Scott A. Yeoman
                                            ------------------------------------
                                            Scott A. Yeoman
                                            President, Chief Executive
                                            Officer and Director
                                            (Principal Executive Officer)


                                        By: /s/ Kurt R. Stevenson
                                            ------------------------------------
                                            Kurt R. Stevenson
                                            Senior Vice President and
                                            Chief Financial Officer
                                            (Principal Financial Officer)

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

<TABLE>
<CAPTION>
           Signature                            Title                     Date
           ---------                            -----                     ----
<S>                              <C>                                  <C>


/s/ Scott A. Yeoman              President, Chief Executive Officer   May 5, 2006
------------------------------   (Principal Executive Officer)
Scott A. Yeoman                  and Director


**                               Chairman of the Board                May 5, 2006
------------------------------   and Director
Dennis J. McDonnell


**                               Director                             May 5, 2006
------------------------------
Richard J. Berry


**                               Director                             May 5, 2006
------------------------------
Walter E. Breipohl


**                               Director                             May 5, 2006
------------------------------
Robert J. Doty
</TABLE>


                                        7

<PAGE>

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


**                               Director                             May 5, 2006
------------------------------
I. J. Reinhardt, Jr.


                                 Director                             May 5, 2006
------------------------------
John A. Shinkle


**                               Director                             May 5, 2006
------------------------------
Scott C. Sullivan


**                               Director                             May 5, 2006
------------------------------
John A. Trainor


**By: /s/ Scott A. Yeoman
      ------------------------
      Scott A. Yeoman
      Attorney-in-Fact
</TABLE>


                                        8

<PAGE>

                                  EXHIBIT INDEX

<TABLE>
<CAPTION>
EXHIBIT
NUMBER    DESCRIPTION
-------   -----------
<S>       <C>
5.1       Opinion of Howard & Howard Attorneys, P.C.

5.2       IRS Determination Letter

23.1      Consent of Crowe Chizek and Company LLC

23.2      Consent of Howard & Howard Attorneys, P.C. (contained in their opinion
          filed as Exhibit 5.1).

24        Power of Attorney
</TABLE>


                                        9