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.

Seattle Slew was a dark brown colt with a small white patch of hair by his left rear hoof bred by Ben S. Castleman. He was sired by Bold Reasoning who won the Jersey Derby and Withers Stakes in 1971. His dam My Charmer went on to produce the 2000 Guineas winner Lomond and Seattle Dancer.  Horse owners since the early 1970s, Karen Taylor was a former flight attendant, and her husband, Mickey Taylor, was a lumberman. They lived in White Swan, Washington. Their friend Dr. Jim Hill, a veterinarian, recommended that they buy Seattle Slew, a son of Bold Reasoning out of the mare My Charmer, at a 1975 Fasig-Tipton yearling auction. Seattle Slew was foaled at Ben Castleman's White Horse Acres Farm near Lexington, Kentucky. Hill and his wife, Sally, had met the Taylors through the horse business. In partnership, they bought 13 prospects, including Seattle Slew, who was sold for $17,500 (equivalent to $80,000 in 2017). They named him for the city of Seattle and the sloughs which loggers once used to transport heavy logs. Karen felt that the spelling of slough--a slow-moving channel of the Pacific Northwest--would be too hard for people to remember, so the spelling was changed to Slew. A later co-owner was Glenn Rasmussen, the accountant for the equine partnerships.  Seattle Slew's owners sent the colt to Billy Turner, a friend and former steeplechase rider who had trained horses seasonally in Maryland since the early 1960s. Based at Belmont Park in the mid-1970s, Turner accepted Seattle Slew and another Taylor-Hill purchase and sent them to Andor Farm in Monkton, where his wife at the time, Paula, taught yearlings to be ridden.  At maturity, he reached 16 hands (64 inches, 163 cm) high.

When was his first race?