Sodium (1963–1983) was an Irish-bred, British-trained Thoroughbred racehorse and sire best known for winning the classic St Leger Stakes in 1966. After running well without winning in 1965 he improved to become one of the best European colts of his generation in 1966 when he developed a rivalry with Charlottown. Sodium finished fourth behind Charlottown in the Epsom Derby but reversed the form to win both the Irish Derby and St Leger. He failed to reproduce his best form as a four-year-old and was retired to stud, where he had little success as a sire of winners in France and Japan.