The 21st Battalion was an infantry battalion of the Australian Army. It was raised in 1915 as part of the First Australian Imperial Force for service during World War I and formed part of the 6th Brigade, attached to the 2nd Division. It fought during the Gallipoli campaign and on the Western Front before being disbanded in late 1918. The battalion was the first Australian battalion to commence active operations on the Western Front and also had the distinction of being the last to pull back when the Australian Corps was withdrawn from the line. In 1921 the battalion was re-raised as a part-time unit of the Citizens Force but was later amalgamated with the 23rd Battalion in 1929 to form the 23rd/21st Battalion.