The 21st Battalion (Eastern Ontario), CEF was an infantry battalion of the Canadian Expeditionary Force in the Great War. The Battalion was authorized on 7 November 1914 and embarked for Britain on 6 May 1915. It disembarked in France on 15 September 1915, where it fought as part of the 4th Infantry Brigade, 2nd Canadian Division in France and Flanders. The battalion was disbanded on 30 August 1920. The 21st Battalion recruited in Eastern Ontario and was mobilized at Kingston, Ontario.