SS Cathay was a P&O passenger steamship that was built in Scotland in 1925 and sunk in the Mediterranean Sea in 1942. In the Second World War she was first an armed merchant cruiser and then a troop ship. In 1942 she took part in Operation Torch, and was sunk in a German air raid off Bougie, Algeria. Cathay was the first of a class of three ocean liners for P&O, all built at the same time. Her sister ships were Comorin and Chitral.