The world's richest horse race just got richer, with organisers announcing a $US16 million ($A21.13 million) purse for the 2018 Pegasus World Cup Invitational in Florida.

The inaugural race in January was worth $12 million, eclipsing the $10 million on offer at the Dubai World Cup.

Dubai ruler Sheikh Mohammed bin Rashid al-Maktoum recently told CNN he wanted the Dubai race to regain its status as the richest race.

"The 2017 Pegasus World Cup Invitational surpassed our expectations and we believe took the sport of thoroughbred racing to an exciting new level," Belinda Stronach, president of the Stronach Group, said.

American colt Arrogate is the reigning champion of both races. He won the Pegasus World Cup at Gulfstream Park in Hallandale Beach on January 28, before taking the Dubai World Cup on March 25.

The Florida event, the brainchild of businessman Frank Stronach, is funded largely by a $1 million entry fee paid by the connections of each of the 12 starting horses.

The Stronach Group will contribute the remaining $4 million to the 2018 race to be held on January 27.