The Royal Navy's T class of diesel-electric submarines was designed in the 1930s to replace the O, P, and R classes. Fifty-three members of the class were built just before and during the Second World War, where they played a major role in the Royal Navy's submarine operations. Four boats in service with the Royal Netherlands Navy were known as the Zwaardvisch class.

HMS Triumph (N18) was a T-class submarine of the Royal Navy. She was laid down by Vickers at Barrow-in-Furness and launched in 1938. The boat was lost in transit in 1942, with a crew of 64, and its fate was unknown until the sunken boat was rediscovered in June 2023.