MainSource Bank is a community bank located in Greensburg, Indiana. The company is the operating subsidiary of the MainSource Financial Group. It operates banks in Indiana, Illinois, Kentucky, and Ohio.

The United States Treasury purchased $57 million in shares of MainSource Financial Group under the Troubled Asset Relief Program. The Treasury sold all of its shares in the company in March 2012.

In February 2002, Indiana United Bancorp announced plans to consolidating the holding company's many separate banking units throughout the state of Indiana that operated under various names into a single bank to be named MainSource Bank, starting with the merger of the two largest units, People's Trust Co. of Brookville, and Union Bank of Greensburg in March to form an operating unit with 35 banking offices in 14 Indiana counties. The other subsidiary banks were also merged into MainSource as soon as the necessary regulatory approvals were obtained. At the same time, it was announced that the name of holding company was going to be changed to MainSource Financial Group, which was completed in May with its Nasdaq trading symbol changed from IUBC to MSFG.

In March 2005, MainSource Financial Group announced the pending acquisition of the Madison-based Madison Bank & Trust Company from National City Corporation for an undisclosed amount. The acquisition was completed in August 2005.

In the early 2010s, MainSource Financial Group expanded into new territories within Southern Indiana by opening new branches in Columbus, Seymour and Bloomington.

On July 25 2017, MainSource Financial Group and First Financial Bancorp entered into a definitive merger agreement. MainSource is expected to merge into First Financial in the first quarter of 2018.