Alton is a city on the Mississippi River in Madison County, Illinois, United States, about 18 miles north of St. Louis, Missouri. The population was 25,676 at the 2020 census. It is a part of the River Bend area in the Metro-East region of the Greater St. Louis metropolitan area. It is famous for its limestone bluffs along the river north of the city, as the former location of the state penitentiary, and for its role preceding and during the American Civil War.