American Pharoah has ended US racing's 37-year Triple Crown drought, romping to victory in the Belmont Stakes.

The free-striding bay colt ridden by Victor Espinoza and trained by Bob Baffert added the 1-1/2-mile (2400m) Belmont to his Kentucky Derby and Preakness wins to become the 12th horse to sweep all three, and the first since Affirmed in 1978.

Since then 13 horses had claimed the first two jewels of the Triple Crown only to be undone by the Belmont.

That list of near-misses included three horses trained by Baffert and two ridden by Espinoza.

"Wow! Wow!" Espinoza said as his mount cantered toward the winner's circle to delirious cheers.

"It's just an amazing thing. He's just an amazing horse!"

American Pharoah came out of the gate slowly but responded quickly as Espinoza took him straight to the lead.

He stayed in front the rest of the way, the first favourite to win the Belmont since Afleet Alex in 2005.

"He's a great horse. It takes a great horse to do it," Baffert said.

The 62-year-old trainer has known Triple Crown heartbreak, having saddled Silver Charm (1997), Real Quiet (1998) and War Emblem (2002) for Kentucky Derby and Preakness wins, only to see them fall short in the Belmont.

"I'm thinking about my parents," he said. "I wish they were alive to see this."

Espinoza was aboard War Emblem for that failed Triple try in 2002, and he had another Triple Crown chance go awry at Belmont on California Chrome last year.

This time, Espinoza said, he knew at the first turn that Pharoah wouldn't let him down.

"He broke a step slow, but in two jumps I was in the lead," Espinoza said.

"I had the best feeling ever when he went to the first turn."

Godolphin-owned Frosted, trained by Kiaran McLaughlin and ridden by Joel Rosario was second. Keen Ice, trained by Dale Romans and ridden by Kent Desormeaux, was third.

American Pharoah ended a record Triple Crown drought. The previous record gap was 25 years, from Citation in 1948 to Secretariat in 1973.

"New Yorkers, all racing fans, this is for you," American Pharoah's owner Ahmed Zayat said.

The field of eight was the smallest in the Belmont since 2007, when the filly Rags to Riches beat six rivals.

US TRIPLE CROWN WINNERS AND JOCKEYS:

2015-American Pharoah (Victor Espinoza)

1978-Affirmed (Steve Cauthen)

1977-Seattle Slew (Jean Cruguet)

1973-Secretariat (Ron Turcotte)

1948-Citation (Eddie Arcaro)

1946-Assault (Warren Mehrtens)

1943-Count Fleet (John Longden)

1941-Whirlaway (Eddie Arcaro)

1937-War Admiral (Charles Kurtsinger)

1935-Omaha (William Saunders)

1930-Gallant Fox (Earl Sande)