Company: ASB
Filing Date: 2025-12-04
Form Type: 425
Source: 0001193125-25-307510
Chunk: 9

Company: ASSOCIATED BANC-CORP
Filing Date: 2025-12-04
Form: 425
Chunk 9
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 the registration statement, as well as other filings containing information about Associated, without charge, at the SEC’s website (http://www.sec.gov) and Associated’s
website (https://investor.associatedbank.com/financials/sec-filings/default.aspx). Copies of the registration statement, when available, and the filings with the SEC that will be incorporated by reference in
the registration statement can also be obtained, without charge, by directing a request in writing to Associated Banc-Corp, Attn: Investor Relations, 433 Main Street, Green Bay, WI 54301 or by email to investor.relations@associatedbank.com.
Reference to Associated’s website does not constitute incorporation by reference of the information contained on the website and is not, and should not be, deemed part of this filing.

No Offer or Solicitation

This communication is for
informational purposes only and is not intended to and does not constitute an offer to subscribe for, buy or sell, or the solicitation of an offer to subscribe for, buy or sell, or an invitation to subscribe for, buy or sell any securities in any
jurisdiction, nor shall there be any sale, issuance or transfer of securities in any jurisdiction in which such offer, invitation, sale or solicitation would be unlawful prior to registration or qualification under the securities laws of any such
jurisdiction. No offer of securities shall be made except by means of a prospectus meeting the requirements of Section 10 of the Securities Act, and otherwise in accordance with applicable law.

7

SIGNATURES Pursuant to the requirements of the Securities Exchange Act of 1934, the registrant has duly caused this report to be signed on its behalf by the undersigned hereunto duly authorized.

| Date: December 4, 2025 |     | Associated Banc-Corp                                              
 (Registrant)                                                      
 By: /s/ Randall J. Erickson                                       |
|:-----------------------|:----|:------------------------------------------------------------------|
|                        |     | Randall J. Erickson                                               |
|                        |     | Executive Vice President, General Counsel and Corporate Secretary |

8

Exhibit 2.1

Execution Version

AGREEMENT AND PLAN OF MERGER

by
and between

ASSOCIATED BANC-CORP

and

AMERICAN NATIONAL
CORPORATION

DATED AS OF NOVEMBER 30, 2025

TABLE OF CONTENTS

| ARTICLE I                                     |     |                                                    |     |    |
| THE MERGER                                    |     |