Justify won the 144th running of the Kentucky Derby at Churchill Downs in Louisville, Kentucky on a sloppy track on Saturday to become the first horse without a start as a two-year-old to win the race since 1882.

Good Magic finished second, according to unofficial results, in the first leg of US thoroughbred racing's Triple Crown series for three-year-olds, with Audible third.

Aidan O'Brien's Mendelssohn finished last. 

Justify, the 5/2 favourite, rocketed out of the gate and was neck and neck with longshot Promises Fulfilled for the first half of the 1 1/4 mile race before veteran jockey Mike Smith pushed the Chestnut colt into gear down the stretch for a comfortable win.

The 'Run for the Roses' victory is the fifth for trainer Bob Baffert and marks the sixth consecutive year where the favourite won.

Justify also became the first horse in 136 years to win the Derby without having a start as a two-year-old. Apollo was the last to do it in 1882.

"It will take a horse like him to break that curse and just wow, he's just something else," said jockey Smith, who also won the race in 2005 on Giacomo.

"I can't describe how special this horse is. My vocabulary doesn't have the words for it," he said of Justify, which has won all four of its starts.

The win came on the rainiest day in the race's history and was watched by 157,813 at the track.