Seattle Slew was a champion American Thoroughbred racehorse who became the tenth winner of the American Triple Crown (1977). He is one of only two horses to have won the Triple Crown while being undefeated in any previous race; the second was Justify who won the Triple Crown in 2018 and is descended from Seattle Slew. Seattle Slew was the 1977 Horse of the Year and a champion at ages two, three, and four. In the Blood-Horse magazine List of the Top 100 U.S. Racehorses of the 20th Century Seattle Slew was ranked ninth.

In the United States, the Triple Crown of Thoroughbred Racing, commonly known as the Triple Crown, is a series of horse races for three-year-old Thoroughbreds, consisting of the Kentucky Derby, Preakness Stakes, and Belmont Stakes. The three races were inaugurated in different years, the last being the Kentucky Derby in 1875. The Triple Crown Trophy, commissioned in 1950 but awarded to all previous winners as well as those after 1950, is awarded to a horse who wins all three races and is thereafter designated as a Triple Crown winner. The races are traditionally run in May and early June of each year, although global events have resulted in schedule adjustments, such as in 1945 and 2020.