UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

WASHINGTON, D.C. 20549

FORM 10-K/A

(Amendment No. 1)

For the fiscal year ended December 31, 2012

For the transition period from                 to                

Commission file number 000-24630

MIDWESTONE FINANCIAL GROUP, INC.

(Exact name of Registrant as specified in its charter)

102 South Clinton Street, Iowa City, IA 52240

(Address of principal executive offices, including zip code)

(319) 356-5800

(Registrant’s telephone number, including area code)

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

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

None

(Title of class)

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

Indicate 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 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.    x

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 the definition of “large accelerated filer,” “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 Act).    ¨  Yes    x   No

The aggregate market value of the voting and non-voting common equity held by non-affiliates of the registrant, based on the last sales price quoted on the NASDAQ Global Select Market on the last business day of the registrant’s most recently completed second fiscal quarter, was approximately $146.7 million.

The number of shares outstanding of the registrant’s common stock, par value $1.00 per share, as of March 5, 2013, was 8,498,484.

Documents Incorporated by Reference

Portions of the registrant’s Proxy Statement for the 2013 Annual Meeting of Shareholders of MidWestOne Financial Group, Inc., to be held on April 18, 2013, are incorporated by reference into Part III of this Annual Report on Form 10-K.

EXPLANATORY NOTE

MidWestOne Financial Group, Inc. (the “Company”) is filing this Amendment No. 1 (the “Amendment”) to its Annual Report on Form 10-K for the year ended December 31, 2012 filed with the Securities and Exchange Commission (the “SEC”) on March 7, 2013 (the “Original Filing”) for the sole purpose of revising the Exhibit Index included with, and incorporated into Item 15 of, the Original Filing. The effect of the revisions to the Exhibit Index will be to incorporate by reference herein corrected versions of certain exhibits which had initially been filed with the Company’s Current Report on Form 8-K on December 27, 2012 (the “Form 8-K”) and which were subsequently corrected on an amendment to such Form 8-K filed with the SEC on March 12, 2013.

In addition, as required by Rule 12b-15 under the Securities Exchange Act of 1934, as amended, new certifications by the Company’s principal executive officer and principal financial officer are filed as exhibits to this Amendment. Except as discussed above, this Amendment does not reflect events occurring after the date of the Original Filing or modify or update any disclosures that may have been affected by subsequent events. Except as discussed above, all other information in the Original Filing remains unchanged.

SIGNATURES

Pursuant to the requirements of Section 13 or 15(d) of the Securities Exchange Act of 1934, the Registrant has duly caused this report to be signed on its behalf by the undersigned, thereunto duly authorized.

INDEX TO EXHIBITS