Julio Mariner (24 January 1975–26 May 2004) was a British Thoroughbred racehorse and sire best known for winning the classic St Leger Stakes in 1978. In a racing career which lasted from August 1977 until October 1978 he won three of his twelve starts. As a two-year-old he won only one of his five races but established himself as a top-class performer when he finished a close and unlucky second in the William Hill Futurity. After running well on his three-year-old debut he produced a series of disappointing efforts before recording an upset victory over a strong field in the St Leger. After his retirement from racing he became a successful sire of sport horses and died in the Netherlands in 2004.