Giant's Causeway, affectionately known as the 'Iron Horse', died on Monday night at the age of 21, Coolmore has announced.
The brilliant six-time Group One winner, who was trained by Aidan O'Brien, had been suffering from "a brief illness" at his Kentucky birthplace, Ashford Stud.
Giant's Causeway was never out of the first two in 13 career starts, and went unbeaten as a juvenile in 1999.
Even better was to come as he rattled off five consecutive Group One triumphs - in the St James's Palace Stakes, the Coral-Eclipse, the Sussex Stakes, the Juddmonte International and the Irish Champion Stakes - as a three-year-old.
His final outing came in the Breeders' Cup Classic at Churchill Downs when, on what his first start on dirt, he was agonisingly denied by Tiznow in an epic encounter.
The son of Storm Cat stood for one season at Coolmore Ireland before he relocated to Ashford.
A statement issued by Coolmore read: "Throughout his 18 years at stud, Giant's Causeway sired an incredible 31 Group One winners worldwide, and will leave behind a long-lasting legacy as a successful sire of sires and top broodmare sire."
Coolmore America manager Dermot Ryan added: "Giant's Causeway was a once-in-a-lifetime horse, and I would like to thank the owners, the Magnier and Tabor families, for entrusting us with his stud career.
"He will be a huge loss not just to us, but to the industry as a whole."