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 well known for its limestone bluffs along the river north of the city. It the former location of an historical state penitentiary, and played a significant role preceding and during the American Civil War.