She was sunk with all hands by Japanese aircraft on 19 January 1945, and was the last Royal Navy submarine to be lost to enemy action.

At least 300 PoWs were killed, and the Germans beached the merchant ship at Methoni in Greece to prevent her sinking and further loss of life.

Towards the end of the year she sank the Italian tanker Giulio Giordani and the auxiliary patrol vessel F-39 / Fertilia, several days after an earlier attack on the ship had failed.

The Italian torpedo boat Generale Antonio Cantore struck a mine laid by Porpoise and was also sunk.

On its return from Operation Rimau, Porpoise is noted to have encountered a small enemy convoy and later a large tanker.

[4] Japanese records show that radio direction finders at Penang picked up her position and subchaser CH-8 was dispatched to the area and conducted an attack on Porpoise.

The aircraft scored a hit with one of its bombs and the submarine sank, bringing up a large oil slick.