SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM 10-K/A

Amendment No. 1

For the fiscal year ended December 31, 2012

OR

For the transition period from                      to                     

Commission File Number: 0-18415

Isabella Bank Corporation

(Exact name of registrant as specified in its charter)

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

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

Common Stock - No Par Value

(Title of Class)

Indicated by check mark if the registrant is a well-known seasoned issuer, as defined in Rule 405 of the Securities Act.    ¨  Yes    x  No

Indicated by check mark if the registrant is not required to file reports pursuant to Section 13 or Section 15(d) of the Act.    ¨  Yes    x  No

Indicate by check mark whether the registrant (1) has filed all reports required to be filed by Section 13 or 15(d) of the Securities Exchange Act of 1934 during the preceding 12 months (or for such shorter period that the registrant was required to file such reports), and (2) has been subject to such filing requirements for the past 90 days.    x  Yes    ¨  No

Indicate by check mark whether the registrant has submitted electronically and posted on its corporate Web site, if any, every Interactive Data File required to be submitted and posted pursuant to Rule 405 of Regulation S-T (Section 232.405 of this chapter) during the preceding 12 months (or for such shorter period that the registrant was required to submit and post such files).    x  Yes    ¨  No

Indicate by check mark if disclosure of delinquent filers pursuant to Item 405 of Regulation S-K is not contained herein, and will not be contained, to the best of registrant’s knowledge, in definitive proxy or information statements incorporated by reference in Part III of this Form 10-K or any amendment to this Form 10-K.  ¨

Indicate by check mark whether the registrant is a large accelerated filer, an accelerated filer, a non-accelerated filer, or a smaller reporting company. See definition of “accelerated filer”, “large accelerated filer” and “smaller reporting company” in Rule 12b-2 of the Exchange Act (Check One).

Indicate by check mark whether the registrant is a shell company (as defined in Rule 12b-2 of the Exchange Act).    ¨  Yes    x  No

The aggregate market value of the voting stock held by non-affiliates of the registrant was $188,923,000 as of the last business day of the registrant’s most recently completed second fiscal quarter.

The number of shares outstanding of the registrant’s Common Stock (no par value) was 7,669,684 as of March 7, 2013.

DOCUMENTS INCORPORATED BY REFERENCE

(Such documents are incorporated herein only to the extent specifically set forth in response to an item herein.)

EXPLANATORY NOTE

This Form 10-K/A is being filed as Amendment No.1 to the Form 10-K of Isabella Bank Corporation filed with the U.S. Securities and Exchange Commission on March 14, 2013 (the “Original Form 10-K”) for the purpose of correcting the cover page to note that disclosure of delinquent filers will be contained in the registrant’s definitive proxy statement. There are no other changes to the Original Form 10-K.

Signatures

Pursuant to the requirements of Section 13 or 15(d) of the Exchange Act, the registrant has duly caused this annual report to be signed on its behalf by the undersigned, thereunto duly authorized.

ISABELLA BANK CORPORATION

(Registrant)

2

Isabella Bank Corporation

FORM 10-K/A

Index to Exhibits

3