HMS Theseus /ˈtʰeː.seu̯s/, [ˈt̪ʰeːs̠ɛu̯s̠] (R64) was a Colossus-class light fleet aircraft carrier of the Royal Navy. Theseus was laid down on 6 January 1943 by Fairfield at Govan and was launched on 6 July 1944. She was involved in the Korean War and the Suez Crisis. The scrapping of the Theseus was commenced on 29 May 1962 at Inverkeithing, Scotland.

The ship's name, Theseus, is derived from a hero in Greek mythology, specifically the legendary king and founder of Athens, who bears the same name.

Theseus was laid down to serve in the Second World War, but was not completed until 9 February 1946, after peace was declared in 1945. Theseus was then utilized as a training vessel until the outbreak of the Korean War.

In 1946, Theseus embarked on work-up and embarked aircraft, conducted trials, and a further work-up for operational service. After preparation for duty in the Far East, she sailed to join the British Pacific Fleet at Singapore as Flagship for the Flag Officer Air, Far East. In 1947, she deployed as the Flagship of the 1st Aircraft Carrier Squadron of the British Pacific Fleet. Upon her return to the United Kingdom she was refitted for service in the Home Fleet. She then joined the 3rd Aircraft Carrier Squadron, Home Fleet.

In 1950, with the beginning of the Korean War, the Theseus was deployed to Korea to commence standard carrier operations. Her first operation involved suppressing enemy defences and communications at Chinnampo, among other locations. Her second operational patrol involved only Combat Air Patrols (CAP) because her take-off weight capacity was limited by a dysfunctional catapult, and thus aircraft were unable to be armed with rockets and bombs. Her third operational duty was as part of the Commonwealth Task Force. Theseus sailed with accompanying ships from Sasebo in Japan. The carrier's aircraft launched successful air strikes on bridges, North Korean troops, and other opportune targets. HMS Theseus' operations in the Korean War primarily concentrated on the Chinnampo area, resulting in widespread damage to the area.

During her fourth operational patrol, which began in December of 1950, aircraft from Theseus sighted and fired on numerous land vehicles in the north of the separated country. Many of the targeted vehicles were destroyed. Soon afterwards, Chinese troops became the target of attacks from the carrier's aircraft, which had completed over 1,630 hours in the air and had fired over 1,400 rockets by the completion of their fourth tour.

On 5 January 1951, Theseus began her fifth operational patrol supporting the American 25th Division, who were fighting south of Osan in South Korea. On 15 January Theseus achieved the milestone of 1,000 accident-free landings. The Carrier Air Group (CAG) were awarded the Boyd Trophy for a remarkable operational tour of duty that included many firsts for pilots and aircrew.

Theseus' sixth operational patrol in late January of 1951, included several major and minor accidents. On 26 January, an aircraft appeared to shudder before spinning out of control and into the sea. The C-class destroyer Cossack went to the location of the crash but to no avail. A second aircraft was hit a number of times by anti-aircraft gunfire, forcing the pilot to ditch the aircraft in a valley in an area near Tongduchon-ni. The pilot waited for rescue for about ninety minutes. An American helicopter flew in and picked up the pilot, while other aircraft from Theseus flew a close-air patrol over the area. On 2 February, a Sea Fury burst a tyre while landing, straining the fuselage and bringing the number of accident-free landings to an end at 1,463.