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): June 4, 2015

Associated Banc-Corp

(Exact name of registrant as specified in its charter)

Registrant’s telephone number, including area code: 920-491-7500

(Former name or former address, if changed since last report)

Check the appropriate box below if the Form 8-K filing is intended to simultaneously satisfy the filing obligation of the registrant under any of the following provisions:

Item 3.03. Material Modification to Rights of Security Holders.

Upon issuance of the Series C Preferred Stock (as described in Item 5.03 below) on June 8, 2015, the ability of Associated Banc-Corp (the “Company”) to declare or pay dividends on, or purchase, redeem or otherwise acquire, shares of its common stock will be subject to certain restrictions in the event that the Company fails to pay dividends on its Series C Preferred Stock. These restrictions are set forth in the Articles of Amendment to the Amended and Restated Articles of Incorporation of the Company (the “Articles of Amendment”) establishing the terms of the Series C Preferred Stock. A copy of the Articles of Amendment is attached hereto as Exhibits 3.1 and 4.1 and is incorporated herein by reference.

Item 5.03. Amendment to Articles of Incorporation or Bylaws

On June 4, 2015, the Company filed the Articles of Amendment with the Wisconsin Department of Financial Institutions, setting forth the terms of its 6.125% Non-Cumulative Perpetual Preferred Stock, Series C, liquidation preference $1,000 per share (the “Series C Preferred Stock”). The terms of the Series C Preferred Stock are more fully described in a prospectus supplement dated June 1, 2015 and filed with the Securities and Exchange Commission on June 2, 2015 (the “Prospectus Supplement”) to the prospectus dated March 17, 2015 (the “Prospectus”) as part of the Company’s Registration Statement on Form S-3 (Registration No. 333-202836) filed on March 18, 2015 (the “Registration Statement”). A copy of the Articles of Amendment is attached hereto as Exhibits 3.1 and 4.1 and is incorporated herein by reference.

Item 8.01. Other Events.

Closing of Depositary Share Offering

On June 8, 2015, the Company completed its public offering and sale of 2,600,000 depositary shares (the “Depositary Shares”), each representing a 1/40th interest in a share of the Company’s Series C Preferred Stock, pursuant to an underwriting agreement entered into by the Company and UBS Securities LLC and Citigroup Global Markets Inc., as representatives of the several underwriters named therein, as described in a Form 8-K filed under the Company with the Securities and Exchange Commission on June 3, 2015 (the “Preferred Stock Offering”). The Depositary Shares and the Series C Preferred Stock have been registered under the Securities Act pursuant to the Registration Statement. The Preferred Stock Offering is more fully described in the Prospectus Supplement. A copy of the Deposit Agreement, dated June 8, 2015, among the Company, Wells Fargo Bank, N.A. and the holders from time to time of the Depositary Receipts described therein is attached hereto as Exhibit 4.2, and the form of Depositary Receipt is attached hereto as Exhibit 4.3.

In connection with the issuance and sale of the Depositary Shares, Godfrey & Kahn, S.C. has delivered an opinion to the Company, a copy of which is attached hereto as Exhibit 5.1, that the Depositary Shares and the Series C Preferred Stock have been validly issued and are fully paid and non-assessable. This Current Report on Form 8-K is being filed to incorporate such opinion by reference into the Registration Statement.

Item 9.01. Financial Statements and Exhibits.

(d) Exhibits

The following exhibits are being filed as part of this Report on Form 8-K:

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.

Exhibit Index