Bob Baffert has trained his second Triple Crown winner with Justify leading all the way to take out the final leg, the Belmont Stakes in New York.

The colt became the 13th horse to claim the Kentucky Derby, Preakness Stakes and Belmont three years after the Baffert-trained American Pharoah broke a 37-year drought.

"This never gets old," an emotional Baffert said.

"American Pharoah, he'll always be my first love. For this horse to do it for (jockey) Mike Smith, he deserved something like this. It feels pretty good."

Baffert said Justify was simply the best horse at the track on Saturday (Sunday AEST).

"He could have won every race on the undercard today. He's just that kind of horse," he said.

Gronkowski battled from the back of the pack to cross the line second, almost two lengths behind Justify.

The large and muscular Justify, owned by China Horse Club, Head of Plains Partners, Starlight Racing & WinStar Farm, made 52-year-old Smith the oldest jockey to ever win the Triple Crown.

"This horse ran a tremendous race. He's so gifted he's sent from heaven," Smith said.

"He's just brilliant."

Smith thanked Baffert for giving him the opportunity to ride Justify into the history books.

"He's made a lot of my goals come true but today, he made my dream come true. I appreciate it," he said.

Justify was sired by Champagne Stakes and Florida Derby champion Scat Daddy and was born at Glenwood Farm in Versailles, Kentucky.

From early on, Baffert said it was clear Justify was special.

"I've wanted to see that horse's name up there because we knew he was brilliant from day one," he said.

With the win, Justify, who did not race as a two-year-old, improved to 6-0 in his young career, which started 111 days ago.

Some had wondered whether the colt might show signs of fatigue following a busy schedule but that was not the case as he was never seriously challenged by any of the other nine horses.