21 Corps, 21st Corps, Twenty First Corps, or XXI Corps may refer to:
XXI Corps (German Empire), a unit of the Imperial German Army prior to and during World War I XXI Corps (Ottoman Empire), active during World War I XXI Corps (United Kingdom), active during World War I Indian XXI Corps, active during World War II XXI Corps (India), currently active Indian Army corps 21st Army Corps (Ukraine) XXI Corps (United States), active during World War II XXI Corps (Union Army), active during the American Civil War XXI Mountain Corps (Wehrmacht), active during World War II
See also List of military corps by number 21st Army (disambiguation) 21st Brigade (disambiguation) 21st Division (disambiguation) 21st Regiment (disambiguation) 21 Squadron (disambiguation)