Undefeated boxer Floyd Mayweather and mixed martial arts champion Conor McGregor, two of the most popular athletes in their respective sport, have agreed to a August 26 fight in Las Vegas.
The fight is expected to gross millions of dollars with Mayweather a solid favourite.
Mayweather, 40, retired in 2015 with an unblemished 49-0 professional boxing record while boastful Irishman McGregor, 28, is the UFC's lightweight champion.
"IT'S OFFICIAL!!!" Mayweather wrote on his Twitter account alongside a short video that included still photos of both fighters.
"THE FIGHT IS ON," McGregor tweeted minutes before Mayweather's post.
The two fighters have gone back and forth for nearly a year regarding a potential showdown with both men previously stating they each wanted $US100 million ($A132 million) for the fight.
Mayweather is widely regarded as one of the best defensive fighters of all time, and prides himself on his ability to evade punishment with his skilful movement about the ring while making adjustments against opponents on the fly.
McGregor, the UFC's reigning lightweight champion and former featherweight champion, is known as much for his bravado as for his explosive knockout power inside an MMA octagon.
The Irishman became the first UFC fighter to hold two belts simultaneously when he knocked out Eddie Alvarez in New York last November for the lightweight championship.
The UFC stripped McGregor of his featherweight belt shortly after that fight when he announced he would be taking some time off after he and long-time partner Dee Devlin had their first baby in May.
Known for his devastating striking and highlight-reel knockouts, McGregor is also quick and irrational on his feet, but few would back him to beat one of the most complete boxers of all time.
UFC president Dana White told ESPN: "We've been in negotiations now for a while and to be honest the negotiation went smooth. We got the thing done.
"The impossible deal is now done."
White said the fight would be at 154lbs - light middleweight - and added: "It'll be in a ring, it's a boxing event.
"I talked to Conor McGregor this morning. He is absolutely 100 per cent positive that he wins this fight and I stopped doubting Conor McGregor a long time ago.
"Floyd Mayweather is 40 years old, he's always had problems with southpaws. Conor McGregor is 27 and he is a southpaw. Whenever Conor hits people, they fall.
"I'm not saying Conor McGregor's going to win. I'm not saying Floyd Mayweather's going to win. But the reason Conor McGregor's as big as he is he'll fight anybody, anywhere, anytime.
"He will go after Floyd Mayweather and he will try to knock him out."
A win for Mayweather would move him ahead of former heavyweight great Rocky Marciano, who also retired with a perfect 49-0 career record.