After the beginning of World War I in August 1914, Swiftsure escorted troop convoys in the Indian Ocean until she was transferred to the Suez Canal Patrol in December. After defending the Canal in early 1915 from Ottoman attacks, the ship was then transferred to the Dardanelles in February and saw action in the Dardanelles Campaign bombarding Ottoman fortifications. Swiftsure was assigned to convoy escort duties in the Atlantic from early 1916 until she was paid off in April 1917 to provide crews for anti-submarine vessels. In mid-1918, the ship was disarmed to be used as a blockship during a proposed second raid on Ostend. Swiftsure was sold for scrap in 1920.