Former multi-division boxing world champion Floyd Mayweather will fight UFC lightweight champion Conor McGregor in Las Vegas on August 26.
Both men took to social media to announce the megafight, with McGregor saying 'the fight is on' and Mayweather tweeting, "It's official."
Mayweather confirmed the bout will take place under normal boxing rules despite McGregor having never boxed either amateur or professionally.
The 40-year-old Mayweather announced his retirement after winning his 49th straight bout against Andre Berto also in Las Vegas in September 2015.
Mayweather will reportedly be paid over $US100 million ($A132 million) for the contest, which will be broadcast on pay-per-view networks across the world.
McGregor has become one of the most recognisable mixed martial artists in the world since he won the UFC featherweight title in December 2015.
After splitting two high-profile bouts with Nate Diaz, McGregor knocked out Eddie Alvarez to add the organisation's lightweight title in December last year.
The pair first mooted the idea of a fight in a series of exchanges on social media, in which the Irishman urged Mayweather to step up and sign.
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.
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