Seattle Slew was an American Thoroughbred race horse who won the Triple Crown in 1977 — the tenth of thirteen horses to accomplish the feat. He is one of two horses to have won the Triple Crown while having been undefeated in any previous race; the second was Justify, who won the Triple Crown in 2018 and who is descended from Seattle Slew. Honored as the 1977 Horse of the Year, he was also 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.