Moon Madness (foaled 1983), was a British Thoroughbred racehorse and sire. In a career which lasted from September 1985 until November 1988, he ran twenty-four times and won ten races. He recorded his most important success when winning the Classic St. Leger Stakes as a three-year-old in 1986, the same year in which he also won the King George V Stakes, and the Scottish Derby. He continued to race for the next two seasons in major middle-distance and staying races, with his victories including the Grand Prix de Saint-Cloud, the Geoffrey Freer Stakes the Cumberland Lodge Stakes and the Yorkshire Cup. He later stood as a stallion in Europe and Japan.