HMS Poseidon (P99) was a Parthian-class submarine designed and built by Vickers Shipbuilding and Engineering in Barrow-in-Furness for the Royal Navy, launched on 22 August 1929. She spent most of her short career assigned to the Yellow Sea region, based at the Royal Navy's base at Weihai, China. At about 12:12 on 9 June 1931, while exercising on the surface with the submarine tender Marazion 20 mi (32 km) north of the vessels' base at Weihai, and despite excellent visibility, Poseidon collided with the Chinese merchant vessel SS Yuta.