Justify charged out of the gate and never gave up the lead, powering his way to victory at the Belmont Stakes to become just the 13th horse to win the Triple Crown.
The colt was briefly challenged in the final turn, but down the stretch jockey Mike Smith let him use his strength to run away from the field. Gronkowski finished second, 1 3/4 lengths back, and Hofburg was third.
"This horse ran a tremendous race, he's so gifted," Smith said. "He's sent from heaven. I tell you, it's just amazing. I can't describe the emotions that's going through my body right now."
Justify's victory made Bob Baffert just the second trainer to win the Triple Crown twice. He previously won in 2015 with American Pharoah, the colt that ended a 37-year drought.
With the win, Justify improved to 6-0 in a career that began in February, making him the first Triple Crown winner not to race as a 2-year-old.
"He's so brilliant. Did you see him in the gate, he's standing so still," Smith said. "I thought maybe he's not going to break today. He lifted like he's going 440 yards in Roswell, New Mexico."
Smith, at 52, is the oldest jockey to win the Triple Crown.
The Kentucky Derby and Preakness winner went off a 4/5 favorite for the 1 1/2-mile Belmont, the longest and most grueling of the three-race series. He paid $3.60 for the win, $3.50 to place and $2.80 to show.
He'd won the first two legs of the Triple Crown on sloppy tracks, charging to a 2 1/2-length victory in the 1 1/4-mile Kentucky Derby on May 5, and then pulling out a a half-length win in the 1 3/16-mile Preakness on May 19.
The Associated Press contributed to this report.