Middletown is a city in Butler and Warren counties in southwestern Ohio, formerly in Lemon, Turtlecreek, and Franklin townships. The population was 51,605 at the 2000 census. Middletown continues to grow as it focuses on outward growth from annexation and not redevelopment. Its name comes from its founder, Stephen Vail, believing it to be about halfway between Cincinnati and Dayton on the Miami and Erie Canal. Vail centered the town in Fractional Section 28 of Town 2, Range 4 North.

It was incorporated by the Ohio General Assembly on February 11, 1833, and became a city in 1886. The city is the home of AK Steel Holding Corporation, formerly Armco, a major steel works founded in 1900.

Middletown contains a small municipal airport known as Hook Field. Its airport code is MWO. It is not served by commercial airlines, but is used for general aviation.