Floyd Mayweather Jr. will fight 22-year-old Saul Alvarez in a light-middleweight unification world title bout in Las Vegas on September 14.
Mayweather extended his unbeaten run to 44 bouts earlier this month when he defeated Robert Guererro to earn back the WBC welterweight title.
The American announced the fight on Twitter: "I'm giving the fans what they want."
Alvarez unified the WBC and WBA light-middleweight titles last month when he beat Austin Trout thanks to a unanimous decision.
The young boxer has already had 43 fights, after remarkably making his professional debut in 2005 at the age of 15.
The Mexican is unbeaten in his fights, winning 31 by knockout, although he did draw a fight with Jorge Juarez in 2006.
Mayweather, despite being 14 years older than his opponent, has a very similar record, winning all of his 44 bouts, 26 of those by knockout.
The fight at the MGM Grand is reportedly going to be at a catchweight - or non-standard weight limit - of 152lb, two pounds below the light-middleweight limit.
Despite this, both fighters will put their light-middleweight titles on the line.
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