HMS L20 was a British L class submarine built by Vickers, Barrow-in-Furness. She was laid down on 26 July 1917 and was commissioned on 28 January 1919. L20 was sent to Hong Kong in 1919. She was placed in the reserve flotilla in 1923 in Hong Kong. She was sold for break up on 7 January 1935 in Newport. Her bell is in the Royal Navy Submarine Museum.