Quarter Moon (foaled 31 January 1999) was an Irish Thoroughbred racehorse and broodmare. As a two-year-old in 2001 she showed considerable promise, taking the Group 1 Moyglare Stud Stakes on the second of her two starts. In the following year she failed to win but showed top class form to finish second in four Group 1 races: the Irish 1,000 Guineas, the Epsom Oaks, the Irish Oaks and the Nassau Stakes. After retiring from racing at the end of the year she became a successful broodmare whose foals have included the Pretty Polly Stakes winner Diamondsandrubies.