HMS Thermopylae (P355) was a T-class submarine in service with the United Kingdom's Royal Navy.

Thermopylae was laid down on 26 October 1943 at Chatham Dockyard, and launched on 27 June 1945.

Commissioned after the end of the Second World War, she had a relatively quiet career.

On 15 January 1950 she ran aground on Stevenson Rock, off Skerryvore, Inner Hebrides.

[1] In 1953 she took part in the Fleet Review to celebrate the Coronation of Queen Elizabeth II.