Question: when did seattle slew win the triple crown?
Answer: in 1977, 1977

Seattle Slew ( February 15 , 1974 -- May 7 , 2002 ) was an American Thoroughbred race horse who won the Triple Crown in 1977 -- the tenth of twelve horses to accomplish the feat . He is the only horse to have won the Triple Crown while having been undefeated in any race previous . Honored as the 1977 Horse of the Year , he was also a champion at age 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 .