Leavenworth is one of the 105 counties in the state of Kansas. It was created in 1855, making it one of the original 36 counties of the Kansas Territory. As of the 2010 census the population was 76,227, up from 68,691 in 2000. The county seat and largest city is Leavenworth.
The county has 468.33 square miles (1,213 km2) of area, of which 463.27 sq mi (1,200 km2) is land. It is bordered by Wyandotte County to the east, Johnson County to the southeast, Douglas County to the southwest, Jefferson County to the west, Atchison County to the northwest and Platte County, Nebraska to the north.