Mad Hatter (foaled 1916 in Kentucky) was an American-bred Thoroughbred racehorse who was named U.S. Champion Older Male Horse in 1921.

He was foaled one year earlier than Man o'War and was a full brother to Belmont Stakes winner Mad Play.

Trained by Sam Hildreth and ridden most of the time by the great jockey Earl Sande, Mad Hatter was known as much for his erratic temperament as for his racing ability.

In 1921, Mad Hatter had victories in the Jockey Club Gold Cup and the Metropolitan Handicap and was named U.S. Champion Older Male Horse.

However, out of 177 foals, Mad Hatter sired 23 graded stakes winners such as The Nut, Cocked Hat, Cresta Run, and the champion filly Snowflake.