Leavenworth is the county seat of and the largest city in Leavenworth County, Kansas, United States. Part of the Kansas City metropolitan area, Leavenworth is located on the west bank of the Missouri River, on 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 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. As of the 2020 census, the population of the city was 37,351.