The death came 25 years to the day after Seattle Slew won the Kentucky Derby.
"Seattle Slew passed away in his sleep," read a release issued by Hill 'n' Dale Farm, where Slew recently was moved after a second operation on his spine.
He was the only living Triple Crown winner. "Affirmed," who won the last Triple Crown in 1978, died in January 2001.
Seattle Slew was purchased for the bargain price of $17,500, and set records on his way to being named top two-year-old colt. He led most of his races right from the start, and set records along the way.
As a three-year-old in 1977, he won the Kentucky Derby, Preakness Stakes and Belmont Stakes.
Despite a shaky start in the Derby, he muscled his way through the leaders to finish with a 1-3/4-lengths lead. He had an easier time of it in the Preakness, setting Pimlico's second-fastest race ever and winning by 1½ lengths.
The Belmont was the horse's first muddy track, but not only didn't it seem to make a difference, he won by four lengths.
During his career, he won more than $1.2 million for his owners, and after retiring, sire many other top thoroughbreds.
Slew, who won 14 of 17 lifetime starts and earned $1,208,726, was in failing health the past two years and underwent two delicate spinal fusion operations.
Last month, Slew was a bit wobbly after the second surgery, which was to "normalize," him, owner Mickey Taylor said. The first one, Taylor said last month during an interview, "saved his life."
Slew sired 101 stakes winners. They include 1984 Kentucky Derby winner Swale, A.P. Indy, Capote and Slew o' Gold, and have earned more than $75 million in purses.