Kentucky Derby and Preakness Stakes winner Charismatic has died unexpectedly aged 21.

Charismatic, whose bid for the 1999 Triple Crown ended when he fractured his leg near the finish line in the Belmont Stakes, died on Sunday at Old Friends thoroughbred retirement farm in Kentucky.

Charismatic was found dead in his stall with his cause of death not immediately known.

"We are devastated. CHARISMATIC 1996-2017 died unexpectedly this morning. Our hearts are broken for all who loved him," Old Friends Farm tweeted.

Charismatic, who was born in 1996 and trained by D Wayne Lukas, was named the Eclipse Horse of the Year as the best of three-year-old in 1999.

In the Belmont, Charismatic led with about a furlong (200 metres) to go but he took an awkward step and fractured his left foreleg as Lemon Drop Kid swept by to win. Charismatic finished third.

The chestnut horse retired with a record of five wins from 17 starts and earnings of more than $US2 million before starting his stud career in 2000.

"Right now, everyone is pretty much inconsolable," Old Friends founder and president Michael Blowen said in a statement.

"(Saturday night), at 6:30, he was fine. He was a really tough horse and he deserved a much longer retirement .

"Everyone at Old Friends takes solace from the few great months that this great champion gave us."

Lukas, a Hall of Fame trainer, visited Charismatic in December.

"He looked good enough that you probably could have tacked him up and galloped him," he said.

"He looked terrific."