HMS P556 (pennant number P556), was a S1-class submarine of the British Royal Navy. She was formerly known as USS S-29. USS S-29 was transferred to the Royal Navy at New London on 5 June 1942. She arrived at Gibraltar, via Bermuda, in August 1942 to refit, nominally as part of the 8th Flotilla. She moved to Plymouth in February 1943, for an anti-submarine training role. In November 1943, she moved to Portsmouth for service in the same role. Due to the number of mechanical failures it suffered, it acquired the nickname "Reluctant Dragon".