8-K

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): October 6, 2025

ISABELLA BANK CORPORATION

(Exact name of registrant as specified in its charter)

Registrant’s telephone number, including area code: (989) 772-9471

Not Applicable

(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:

| ☐ | Written communications pursuant to Rule 425 under the Securities Act (17 CFR 230.425) | | --- | --- || ☐ | Soliciting material pursuant to Rule l4a-12 under the Exchange Act (17 CFR 240.l4a-l2) | | --- | --- || ☐ | Pre-commencement communications pursuant to Rule l4d-2(b) under the Exchange Act (17 CFR 240.l4d-2(b)) | | --- | --- || ☐ | Pre-commencement communications pursuant to Rule 13e-4(c) under the Exchange Act (17 CFR 240.l3e-4(c)) | | --- | --- |

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

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-2 of the Securities Exchange Act of 1934 (§240.12b-2 of this chapter).

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. ☐

Section 5 - Corporate Governance and Management

Item 5.02 Departure of Directors or Certain Officers; Election of Directors; Appointment of Certain Officers; Compensatory Arrangements of Certain Officers.

(d)    Appointment of Brian Tessin as a Director for Isabella Bank Corporation and Isabella Bank

On October 6, 2025, the Board of Directors of the Corporation appointed Brian Tessin to the Board of Directors of Isabella Bank Corporation and to the Board of Directors of Isabella Bank. Mr. Tessin will serve a term as a director until the Corporation's Annual Meeting of Shareholders in 2026. The Board has determined that Mr. Tessin is an "independent director" within the meaning of applicable rules and regulations of the Securities and Exchange Commission.

The Corporation issued a press release dated October 9, 2025 announcing Mr. Tessin's appointment to the Board of Directors of Isabella Bank Corporation and to the Board of Directors of Isabella Bank.

Section 9 - Financial Statements and Exhibits

Item 9.01 Financial Statements and Exhibits

(d)    Exhibits:

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.

INDEX TO EXHIBITS

Document

Exhibit 99.1

For Immediate Release

Isabella Bank Corporation

401 N. Main Street

Mt. Pleasant, MI 48858-1649

FOR MORE INFORMATION CONTACT:

Lori Peterson, Vice President and Director of Marketing

Phone: 989-779-6333 | lapeterson@isabellabank.com

Isabella Bank Appoints Financial Executive Brian Tessin

to its Boards of Directors

Mt. Pleasant, MI, October 9, 2025 –Isabella Bank Corporation has appointed Brian Tessin to the Isabella Bank Board and Isabella Bank Corporation Board of Directors, effective October 6, 2025.

Mr. Tessin is the Chief Tax Officer for Dow Inc., overseeing all aspects of tax strategy, planning, compliance, and financial reporting. He has more than 30 years of experience in tax and financial leadership across global organizations.

“We are honored to welcome Mr. Tessin to our Boards of Directors,” said Jerome Schwind, President and Chief Executive Officer. “Brian’s extensive experience in global tax strategy, financial reporting, M&A, and regulatory compliance will be instrumental as we continue to grow and navigate an increasingly complex financial landscape. He knows our communities well, and his leadership and insight also reinforce our commitment to sound governance and fiscal responsibility.”

Prior to rejoining Dow in 2019, Mr. Tessin was Vice President, Tax and Chief Tax Counsel for Albemarle Corporation. Earlier in his career, Mr. Tessin spent nearly two decades at Dow Corning Corporation, ultimately serving as Corporate Vice President of Tax. He began his professional career in public accounting, gaining foundational experience in both tax and assurance.

A Saginaw, Michigan native, Mr. Tessin earned dual Bachelor of Science degrees in Accounting and Finance from Ferris State University, a Juris Doctor degree from Thomas M. Cooley Law School, and a Master of Science degree in Taxation from Walsh College. He is a Certified Public Accountant and is a licensed attorney in the State of Michigan.

He is an active member of several professional organizations, including the State Bar of Michigan, Tax Executives Institute, and serves on tax policy committees for the Financial Executives Institute and the National Association of Manufacturers. Mr. Tessin also has served on the boards of Midland Tomorrow (Midland Business Alliance’s economic development organization), and Wellspring Lutheran Services of Michigan.

“I am excited to join the Isabella Bank Board of Directors,” said Tessin. “Isabella Bank has a long-standing reputation for excellence and community commitment. I look forward to contributing to its continued success and supporting its mission of delivering trusted financial services in our communities.”

Isabella Bank Corporation (NASDAQ: ISBA) is the parent holding company of Isabella Bank, a state-chartered community bank headquartered in Mt. Pleasant, Michigan. Isabella Bank was established in 1903 and has been committed to serving the local banking needs of its customers and communities for

more than 122 years. The Bank offers personal and commercial lending and deposit products, as well as investment, trust, and estate planning services through Isabella Wealth. The Bank has locations throughout eight Mid-Michigan counties: Bay, Clare, Gratiot, Isabella, Mecosta, Midland, Montcalm, and Saginaw.