Aidan O'Brien hailed St Mark's Basilica as "possibly the best horse we have ever had" after Coolmore announced the multiple Group One winner has been retired to stud.
Last year's European champion two-year-old after rounding off his juvenile campaign with victory in the Dewhurst at Newmarket, the Siyouni colt picked up where he left off by claiming Classic glory in the French 2000 Guineas.
He landed another Classic win in the French Derby before beating his elders for the first time in the Coral-Eclipse at Sandown - seeing off subsequent Juddmonte International hero Mishriff in dominant style.
Having missed a rematch with Mishriff at York due to a leg injury, St Mark's Basilica made it four from four for the season in an epic renewal of the Irish Champion Stakes at Leopardstown.
But O'Brien revealed last week his star colt had not yet returned to full work, with the injury suffered pre-Leopardstown again causing problems, and it was confirmed on Monday afternoon he had run his final race.
O'Brien said in a statement posted on the Coolmore website: "St Mark's Basilica is possibly the best horse we have ever had in Ballydoyle.
"He relaxes and quickens and has all the determination from Galileo and all the speed from Siyouni."
Coolmore's David O'Loughlin said: "He's the most exciting prospect we've retired from Ballydoyle since his late grandfather Galileo.
"European Champion Two-Year-Old, the highest-rated three-year-old in the world, a 1.3million guineas yearling by a top sire in Siyouni, a half-brother to another brilliant horse in Magna Grecia, both out of Galileo's group-winning two-year-old Cabaret.
"He has everything and he'll get terrific support from his owners' broodmare band too."