Company: ASB
Filing Date: 2025-12-04
Form Type: 425
Source: 0001193125-25-307510
Chunk: 0

Company: ASSOCIATED BANC-CORP
Filing Date: 2025-12-04
Form: 425
Chunk 0
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UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 FORM 8-K CURRENT REPORT Pursuant to Section 13 OR 15(d) of The Securities Exchange Act of 1934 Date of Report (Date of earliest event reported): November 30, 2025 ASSOCIATED BANC-CORP (Exact name of registrant as specified in its charter)

| Wisconsin                    |     | 001-31343   |     | 39-1098068         |
| (State or other jurisdiction 
 of incorporation)            |     | -Commission 
 File Number |     | -IRS Employer      
 Identification No. |

433 Main Street, Green Bay, Wisconsin 54301 (Address of principal executive offices) (Zip Code) 920 491-7500 (Registrant’s telephone, including area code) Not Applicable (Former name or former address, if changed since last report.) Check the appropriate box below if the Form 8-Kfiling is intended to simultaneously satisfy the filing obligation of the registrant under any of the following provisions (see General Instruction A.2. below):

| ☒ | Written Communications pursuant to Rule 425 under the Securities Act (17 CFR 230.425) |

| ☐ | Soliciting material pursuant to Rule 14a-12 under the Exchange Act (17 
 CFR 240.14a-12)                                                        |

| ☐ | Pre-commencement communications pursuant to Rule 14d-2(b) under the Exchange Act (17 CFR 240.14d-2(b)) |

| ☐ | Pre-commencement communications pursuant to Rule 13e-4(c) under the Exchange Act (17 CFR 240.13e-4(c)) |

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

| Title of each class:                                                                                |     | Trading 
 Symbol  |     | Name of each exchange   
 on which registered:    |
| Common stock, par value $0.01 per share                                                             |     | ASB     |     | New York Stock Exchange |
| Depositary Shrs, each representing 1/40th intrst in a shr of 5.875% Non-Cum. Perp Pref Stock, Srs E |     | ASB PrE |     | New York Stock Exchange |
| Depositary Shrs, each representing 1/40th intrst in a shr of 5.625% Non-Cum. Perp Pref Stock