Company: ASB
Filing Date: 2025-12-04
Form Type: 425
Source: 0001193125-25-307510
Chunk: 1

Company: ASSOCIATED BANC-CORP
Filing Date: 2025-12-04
Form: 425
Chunk 1
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, Srs F |     | ASB PrF |     | New York Stock Exchange |
| 6.625% Fixed-Rate Reset Subordinated Notes due 2033                                                 |     | ASBA    |     | New York Stock Exchange |

Indicate by check mark whether the registrant is an emerging growth company as defined in Rule 405 of the Securities Act of 1933 (§230.405 of this chapter) or Rule 12b-2of the Securities Exchange Act of 1934 (§240.12b-2of this chapter). Emerging growth company ☐ If an emerging growth company, indicate by check mark if the registrant has elected not to use the extended transition period for complying with any new or revised financial accounting standards provided pursuant to Section 13(a) of the Exchange Act. ☐

Item 1.01 Entry into a Material Definitive Agreement.

Agreement and Plan of Merger

Overview

On
November 30, 2025, Associated Banc-Corp, a Wisconsin corporation (“Associated”), entered into an Agreement and Plan of Merger (the “Merger Agreement”) with American National Corporation, a Nebraska corporation
(“American National”). The Merger Agreement provides that, upon the terms and subject to the conditions set forth therein, American National will merge with and into Associated (the “Merger”), with Associated surviving the
Merger (the “Surviving Corporation”). Immediately following the effective time of the Merger (the “Effective Time”) or at such later time and date as Associated may determine in its sole discretion, Associated will cause
American National’s wholly owned banking subsidiary, American National Bank, to merge with and into Associated’s wholly owned banking subsidiary, Associated Bank, National Association (the “Bank Merger”), with Associated
Bank, National Association continuing as the surviving bank in the Bank Merger. The Merger Agreement was unanimously approved by the Board of Directors of each of Associated and American National.

Upon the terms and subject to the conditions set forth in the Merger Agreement, at the Effective Time, each share of voting stock, par value
$1.00 per share, and each share of non-voting stock, par value $0.01 per share, of American National issued and outstanding immediately prior to the Effective Time, other than certain excepted shares
(including those held by American National or Associated, if any), will be converted into the right to receive 36.250 shares (the “Exchange Ratio,” and such shares,