After sitting two and three deep from his wide gate American Pharaoh out stayed Firing Line to win by a length with Dortmund, a stablemate of the winner, a gallant third after setting the pace.

American Pharoah is the first US 2YO champion to win the Derby since Street Sense in 2007and joins Silver Charm (1997), Real Quiet (1998) and War Emblem (2002) as the Baffert-trained winners of America’s most coveted race.

Winning jockey Victor Ezpinoza Espinoza made it back-to-back Derby wins after last year’s victory aboard California Chrome. He also won on War Emblem for Baffert in 2002 and is the seventh jockey to win three Kentucky Derbys.

The colt is raced by his breeder Ahmed Zayat, who has previously seen his runners finish second in the Kentucky Derby three times including American Pharaoh’s sire Pioneerof the Nile in 2011.

The three placegetters raced in the first four from the start and broke away from the field on the final turn.

Firing Line, with Gary Stevens up, got the better of Dortmund but then had to contend with American Pharoah coming on strongly on the outside. They came together briefly when shifting ground before the favourite gained the upper hand after a brief struggle.

"We were ready to rumble," Baffert said. "I felt very confident going in."

Espinoza said Baffert had given him a lot of confidence.

"I feel like the luckiest Mexican on earth. He has been a special horse since the first time I rode him,” the jockey said.

“Turning for home I started riding a little bit harder. I just couldn't put that other horse (Firing Line) away, but he got it done."

American Pharoah’s 2YO season in 2014 included wins in the G1 Del Mar Futurity and G1 FrontRunner Stakes at Santa Anita but he missed the Breeders' Cup with an injury.

American Pharoah returned to win the G2 Rebel Stakes at Oaklawn Park and then won the $1 million Arkansas Derby by eight lengths.

He has now won five of his six career starts and $US2,830,300.