Filed Pursuant to Rule 424(b)(3) Registration No. 333-220583 MERGER PROPOSED—YOUR VOTE IS VERY IMPORTANT Dear Shareholder:Merger Agreement • May 5th, 2020 • Ohio
Contract Type FiledMay 5th, 2020 JurisdictionOn July 25, 2017, First Financial Bancorp., or First Financial, and MainSource Financial Group, Inc., or MainSource, entered into an Agreement and Plan of Merger (which we refer to as the “merger agreement”) that provides for the combination of the two companies. Under the merger agreement, MainSource will merge with and into First Financial, with First Financial as the surviving corporation, in a transaction we refer to as the “merger.” Following the completion of the merger, MainSource Bank, a wholly-owned bank subsidiary of MainSource, will merge with and into First Financial Bank, a wholly-owned subsidiary of First Financial, with First Financial Bank as the surviving bank, in a transaction we refer to as the “bank merger.” The merger will combine two Midwest community banks to create a $13 billion institution with scale and strength in commercial and retail banking, wealth management, and specialty finance.