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.
A win for Mayweather would move him ahead of former heavyweight great Rocky Marciano, who also retired with a perfect 49-0 career record.
Mayweather will not be the first boxing champion to take on challenges from opponents steeped in other combat sports.
Muhammad Ali fought a 15-round draw against Japanese professional wrestler Antonio Inoki in Tokyo in 1976, in which each man was allowed to use his own respective talent.