121st

121st may refer to:


 * 121st (Western Irish) Battalion, CEF, a unit in the Canadian Expeditionary Force during the First World War
 * 121st Air Refueling Wing (121 ARW) is an aerial refueling organization assigned to the Ohio Air National Guard
 * 121st Engineer Battalion, one of the first American units to land in Normandy on D-Day during World War II
 * 121st Fighter Squadron flies the F-16C/D Fighting Falcon
 * 121st Infantry Division (German Empire), a unit of the Imperial German Army in World War I
 * 121st New York Volunteer Infantry Regiment, a volunteer regiment recruited during the American Civil War
 * 121st Pennsylvania Regiment, a regiment of the Union Army during the United States Civil War
 * 121st Regiment of Foot (1762), an infantry regiment of the British Army, formed in 1762 and disbanded in 1764
 * 121st Regiment of Foot (1794), an infantry regiment of the British Army, formed in 1794 and disbanded in 1796
 * 121st Signal Battalion (United States), a signal unit of the United States Army, inactivated as of July 2006
 * 121st Weather Flight, a subordinate unit of the 113th Wing of the United States Air Force based at Andrews Air Force Base, Maryland