Diadem (1914–1931) was a British Thoroughbred racehorse who won the 1000 Guineas in 1917. She went on to become a top sprinter, recording two victories in both the King's Stand Stakes and the July Cup. In total she won 24 of her 39 races. Diadem was bred and owned by Edgar Vincent, 1st Viscount D'Abernon, and trained by George Lambton. The Diadem Stakes at Ascot Racecourse was named after her.