Madison County is a county located in the central portion of the U.S. state of Ohio. As of the 2020 census, the population was 43,824. Its county seat is London. The county is named for James Madison, President of the United States and was established on March 1, 1810.

London is a city in and the county seat of Madison County, Ohio, United States. Located about 25 miles (40 km) southwest of the Ohio capital of Columbus, London was established in 1811 to serve as the county seat. The population was 10,279 at the 2020 census. It is part of the Columbus metropolitan area.