Trainer Bob Baffert said he and the horse's owners couldn't determine if Justify would be healthy enough to race by the fall and instead chose to call it a career for the historic horse.
"He is just not responding quick enough for a fall campaign," Baffert said. "We all wanted to see Justify run again, but ultimately it is my responsibility to make sure he is perfect. Without 60-90 days, I can't be definite."
"The timing is bad for another start in 2018, and therefore, we have to retire him," WinStar Farm's Elliott Walden, a co-owner of the horse, added. "Like American Pharoah, we can't take the risk of running Justify as a four-year-old. We all wanted him to finish his career in the Breeders' Cup Classic, but it was not meant to be."
Justify captivated horse racing enthusiasts this spring, joining American Pharoah (2015) as just the second horse since Affirmed in 1978 to win all three of the Kentucky Derby, Preakness and Belmont Stakes.
Beyond the Triple Crown, Justify won all six of his races and became the first horse since Apollo in 1882 to win the Kentucky Derby despite not racing as a two-year-old. He totaled $3.8 million in career earnings and will bring home far more in breeding fees.
And despite an inability to race as a four-year-old, Justify's legacy is secure.
"There was never a time when I rode him that I felt like I was going to get beat," jockey Mike Smith told the AP. "There was no horse who could run with him without sacrificing themselves, and there was no horse who could come get him."
In the end, only a bum ankle could defeat Justify.