Rhadamanthus was a British Thoroughbred racehorse and sire. In a career that lasted from April 1790 to 1793 he ran nine times and won five races. In the summer of 1790 he proved himself one of the best British colts of his generation, winning The Derby while still undefeated. He raced until the age of six, winning three further races. All of his runs apart from the Derby were at Newmarket.