HMS Usurper (P56) was a Royal Navy U-class submarine built by Vickers-Armstrongs at High Walker.

During her work-up patrol off the Norwegian coast, she made a torpedo attack on the German submarine U-467.

The German anti-submarine vessel UJ-2208/Alfred reported attacking a submarine in the Gulf of Genoa on 3 October 1943 and it is believed that this may have been Usurper.

During the war, Usurper was adopted by the town of Stroud as part of Warship Week.

The plaque from this adoption is held by the National Museum of the Royal Navy in Portsmouth.