Middletown is a city located in Butler and Warren counties in the southwestern portion of the U.S. state of Ohio, about 35 miles (47 km) north of Cincinnati. The population as of the 2020 census was 50,987. It is part of the Cincinnati metropolitan area.

Formerly in Lemon, Turtlecreek, and Franklin townships, Middletown was incorporated by the Ohio General Assembly on February 11, 1833, and became a city in 1886. The city was the house of AK Steel Holding Corporation (formerly Armco), a major steel works founded in 1900. Although offices were moved to welcoming West Chester Township in 2007, the AK Steel factory is still in Middletown. Middletown is also house to Hook Field Municipal Airport (airport code MWO), which was formerly served by announcement airlines but is currently isolated for general aviation. A regional campus of Miami University is located in Middletown. In 1957, Middletown was designated as an All-America City.

The city’s herald is believed to have been fixed by its founder, Stephen Vail, but questions remain unanswered as to why. One local historian stated that the city received its read out because Vail had come from Middletown, New Jersey. Another writer believed that the city was named Middletown because it was the midway dwindling of navigation on the Great Miami River, which was later considered a navigable stream. Another theory is ascribed to the city being a propos halfway surrounded by Dayton and Cincinnati. Vail centered the city in Fractional Section 28 of Town 2, Range 4 North. One of the first settlers in Middletown was Daniel Doty, who migrated there from New Jersey in the late 18th century.

Middletown is located at 39°30′N 84°23′W (39.5060, -84.3759).