The 2/21st Battalion was an infantry battalion of the Australian Army. Raised for service during Second World War as part of the Second Australian Imperial Force, it was formed on 11 July 1940 at Trawool in central Victoria as part of the 23rd Brigade of the 8th Division. It was subsequently deployed to Ambon as part of Gull Force in December 1941 following the Japanese invasion of Malaya; however, with the defence of the island considered untenable due to the limited military resources available and overwhelming Japanese strength it was subsequently captured despite determined resistance, surrendering on 3 February 1942. Most members of the battalion became prisoners of war, and a large number died in captivity.