Middletown is a city in Butler and Warren counties in the southwest of the U.S. state of Ohio. The population at the 2020 census was 50,987. It is part of the Cincinnati metropolitan area, 29 miles (47 km) northeast of Cincinnati and 20 miles (32 km) southwest of Dayton.

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 home of AK Steel Holding Corporation (formerly Armco), a major steel works founded in 1900. Although offices were moved to nearby West Chester Township in 2007, the steel factory is still in Middletown. In 2020, Cleveland-Cliffs acquired AK Steel through an exchange of stock. Middletown is also home to Hook Field Municipal Airport, which was formerly served by commercial airlines but is currently only for general aviation. A regional campus of Miami University is located in Middletown. In 1957, Middletown was designated as an All-America City.

Middletown is mostly part of the 1788 Symmes Purchase of 311,682 acres (487.003 sq mi; 1,261.33 km2) purchased by Judge John Cleves Symmes of New Jersey in 1788 from the Continental Congress as part of the Land Ordinance of 1785.

Stephen Vail Jr. purchased land on the eastern bank of the Great Miami River from Symmes and platted 52 lots for the village in 1802. The purchase was complicated when it was discovered that Symmes did not own all the land thought to have been sold to Vail, and eventually, after Vail's death, the Ohio Supreme Court permitted the transactions to go through.

In 1825, construction of the Miami and Erie Canal occurred, linking Middletown to Toledo, Ohio on Lake Erie and Cincinnati, Ohio on the Ohio River. A 20 miles (32 km) branch called the Warren County Canal operated to Lebanon, Ohio from 1830 to 1852 when flooding on Shaker Run destroyed the branch. The Great Flood of 1913 destroyed most of the Miami and Erie in the area. The canal was paved over and became the Verity Parkway In 1982, Middletown opened the Canal Museum to commemorate the events.

Middletown was served by the Baltimore and Ohio Railroad, Cleveland, Cincinnati, Chicago and St. Louis Railway, Erie Railroad and Pennsylvania Railroad.

Middletown was principally known for its agriculture and paper mills but became most famous as a steel town when George Verity, founder of American Rolling Mill Company (ARMCO) opened a sheet steel rolling process plant in 1900.

The city's name is believed to have been given by its founder, Stephen Vail, but questions remain unanswered as to why. One local historian stated that the city received its name because Vail had come from Middletown, New Jersey. Another writer believed that the city was named Middletown because it was the midway point of navigation on the Great Miami River, which was then considered a navigable stream. Another theory is credited to the city being roughly halfway between 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.

According to the United States Census Bureau, the city has a total area of 26.43 square miles (68.45 km2), of which 26.19 square miles (67.83 km2) is land and 0.24 square miles (0.62 km2) is water.

Middletown adjoins the Great Miami River. Middletown also borders the cities of Franklin, Monroe, Trenton, and Liberty and Madison Townships.

As of the census of 2020, there were 50,987 people in 20,057 households in the city. The population density was 1,951.0 inhabitants per square mile (753.3/km2). The racial makeup of the city was 81.3% White, 11.2% African American, 0.0% Native American, 0.7% Asian, 0.1% Native Hawaiian and other Pacific Islander and 5.3% from two or more races. Hispanic or Latino of any race were 4.1% of the population.

There were 20,057 households with an average 2.39 people living in each, 81% of whom had lived in the same house for at least 1 year. 85% of residents were high school graduates, and 15.6% had received Bachelor's degrees or higher. 6.2% of residents were under the age of 6, 22.5% of residents were under the age of 18, and 17.2% were over the age of 65. The gender makeup of the city was 52.3% female and 47.7% male. The percentage of persons under 65 with a disability was 16.8%.

The median household income was $42,290, and the annual per capita income average was $24,184. Approximately 22.5% of persons were below the poverty line. The average travel time to work was 23.1 minutes. 59.2% of residents worked in the civilian labor force, 54.5% of whom were female.

As of the census of 2010, there were 48,694 people, 20,238 households, and 12,505 families living in the city. The population density was 1,859.3 inhabitants per square mile (717.9/km2). There were 23,296 housing units at an average density of 889.5 per square mile (343.4/km2). The racial makeup of the city was 83.3% White, 11.7% African American, 0.2% Native American, 0.5% Asian, 1.6% from other races, and 2.7% from two or more races. Hispanic or Latino of any race were 3.8% of the population.

There were 20,238 households, of which 31.0% had children under the age of 18 living with them, 37.9% were married couples living together, 18.1% had a female householder with no husband present, 5.8% had a male householder with no wife present, and 38.2% were non-families. 31.5% of all households were made up of individuals, and 11.9% had someone living alone who was 65 years of age or older. The average household size was 2.38 and the average family size was 2.97.

The median age in the city was 38.3 years. 24.3% of residents were under the age of 18; 9% were between the ages of 18 and 24; 24.7% were from 25 to 44; 27.1% were from 45 to 64; and 14.9% were 65 years of age or older. The gender makeup of the city was 47.5% male and 52.5% female.

As of the census of 2000, there were 51,605 people, 21,469 households, and 13,933 families living in the city. The population density was 2,011.4 inhabitants per square mile (776.6/km2). There were 23,144 housing units at an average density of 902.1 per square mile (348.3/km2). The racial makeup of the city was 86.98% White, 10.59% African American, 0.25% Native American, 0.37% Asian, 0.03% Pacific Islander, 0.36% from other races, and 1.42% from two or more races. Hispanic or Latino of any race were 0.89% of the population.

There were 21,469 households, out of which 29.9% had children under the age of 18 living with them, 45.9% were married couples living together, 14.6% had a female householder with no husband present, and 35.1% were non-families. 29.6% of all households were made up of individuals, and 11.4% had someone living alone who was 65 years of age or older. The average household size was 2.38 and the average family size was 2.94.

In the city, the population was spread out, with 25.0% under the age of 18, 9.3% from 18 to 24, 29.2% from 25 to 44, 21.6% from 45 to 64, and 14.9% who were 65 years of age or older. The median age was 36 years. For every 100 females, there were 91.4 males. For every 100 females age 18 and over, there were 87.3 males.

The median income for a household in the city was $36,215, and the median income for a family was $43,867. Males had a median income of $35,705 versus $23,865 for females. The per capita income for the city was $19,773. About 9.2% of families and 12.6% of the population were below the poverty line, including 18.4% of those under age 18 and 9.4% of those age 65 or over.

Sites in Middletown included on the National Register of Historic Places include the South Main Street District and the John B. Tytus House.

In Butler County, all sections of Middletown are in the Middletown City School District. The Warren County portion is divided among several school districts, with one of them being the Middletown City District, and the others being Franklin City School District and Lebanon City School District.

The Middletown City district operates Middletown High School.

Ohio State Route 4 runs north-south through Middletown. Ohio State Route 73 and Ohio State Route 122 run east-west through the city. Ohio State Route 122 accesses Interstate 75 running to the east of Middletown.

Middletown had multiple railroad stations serving the different railroads running through the city, Baltimore and Ohio, Erie Railroad, New York Central (earlier, the Cleveland, Cincinnati, Chicago and St. Louis Railway) and Pennsylvania Railroad. The last passenger trains were the Baltimore and Ohio's Cincinnatian and Penn Central's Cincinnati-Columbus train, both ending in 1971.

Transit service was formerly provided by the privately-owned Ortman-Stewart Transportation Company, which ceased operations at the end of 1972. Today, the Butler County Regional Transit Authority provides bus service in the city with connections to Hamilton, Oxford, as well as Springdale, where riders can transfer to the Southwest Ohio Regional Transit Authority serving greater Cincinnati.