John A. Shirreffs (born June 1, 1945 in Fort Leavenworth, Kansas) is an American Thoroughbred racehorse trainer. Based in California, Vietnam War veteran John Shirreffs began training Thoroughbreds in 1978. He has won a number of stakes races with his most important coming in the 2005 Kentucky Derby when Giacomo scored a major upset. In 2007, another Shirreffs-trained horse scored a major upset when Tiago, a half-brother to Giacomo (both out of the mare Set Them Free) won the Santa Anita Derby. He is also the trainer of champion Zenyatta, beaten only once in twenty career starts, and winner of the Breeders' Cup Ladies' Classic in 2008 and Breeders' Cup Classic in 2009.