Sun Castle (1938 – March 1942) was a British Thoroughbred racehorse who raced during World War II and was best known for winning the classic St Leger in 1941. After showing promise as a two-year-old he finished third in the 2000 Guineas the following spring but ran poorly when strongly-fancied for the New Derby. He won a race at Newbury in August before taking the substitute New St Leger at Manchester Racecourse. He died in the following spring after contacting tetanus.