Leavenworth is the county seat and largest city of Leavenworth County, Kansas, United States and is part of the Kansas City metropolitan area. As of the 2020 census, the population of the city was 37,351. It is located on the west bank of the Missouri River. The site of Fort Leavenworth, built in 1827, the city became known in American history for its role as a key supply base in the settlement of the American West. During the American Civil War, many volunteers joined the Union Army from Leavenworth. The city has been notable as the location of several prisons, particularly the United States Disciplinary Barracks and United States Penitentiary, Leavenworth.