Floyd Mayweather will fight Manny Pacquiao in Las Vegas on 2 May in what promises to be the richest bout in the history of boxing.
American Mayweather, 37, and Filipino Pacquiao, 36, are considered to be the best two fighters of their generation.
Mayweather's WBC and WBA welterweight titles and Pacquiao's WBO belt will be on the line at the MGM Grand.
Mayweather is unbeaten in 47 professional fights; Pacquiao has 57 wins and five losses from 64 contests.
The fight is expected to generate as much as £162m ($250m). The most lucrative fight in history is thought to be Mayweather's bout against Saul Alvarez in 2013, which made an estimated £97m.
Mayweather-Pacquiao is also expected to break the record for pay-per-view buys in the United States. The current record of 2.4 million was set when Mayweather fought fellow American Oscar de la Hoya in 2007.
Mayweather wrote on his webpage: "What the world has been waiting for has arrived. Mayweather vs Pacquiao on May 2, 2015 is a done deal.
"This will be the biggest event in the history of the sport. Boxing fans and sports fans around the world will witness greatness on May 2."
Head to head
Floyd Mayweather Manny Pacquiao
Money Nickname Pac Man
Las Vegas, USA Home town General Santos City, Philippines
24 February 1977 (37) Date of birth 17 December 1978 (36)
Orthodox Stance Southpaw
5ft 8in Height 5ft 6in
47 fights, 47 wins (26 KOs) Pro record 64 fights, 57 wins (38 KOs), 5 losses, 2 draws
WBC & WBA welterweight, WBC light-middleweight Current titles WBO welterweight
Pacquiao said: "I am very happy that Floyd Mayweather and I can give the fans the fight they have wanted for so many years.
"They have waited long enough and they deserve it. It is an honour to be part of this historic event.
"I dedicate this fight to all the fans who willed this fight to happen and, as always, to bring glory to the Philippines and my fellow Filipinos around the world."
Mayweather said "giving the fans what they want is always my main focus".
He added: "I am the best ever and this fight will be another opportunity to showcase my skills and do what I do best, which is win.
"Manny is going to try to do what 47 before him failed to do, but he won't be successful. He will be number 48."
Pacquiao's trainer Freddie Roach, said: "Floyd should enjoy being the A-side while he can. Because on May 2, Manny is going to put him on his backside."
Mayweather and Pacquiao met for the first time at a basketball game in Miami in January, when they exchanged telephone numbers.
Mayweather is regarded by many as the finest technician and tactician in boxing, while Pacquiao has built his career on a more aggressive style.
'They will need tax men in their corners'
"This fight is the biggest boxing can offer by some distance. Las Vegas, festooned with giant portraits of the two rivals all along its Strip, will bow to their cults of personality. Fans will flock from far and wide, rollers will be high, celebrities will be A-list, journalists will be varied and many."
Read more from BBC Sport's Ben Dirs
Pacquiao beat Chris Algieri on a unanimous points decision in his most recent fight in November, but lost twice in 2013 - to Timothy Bradley and Juan Manuel Marquez.
Mayweather, whose unbeaten record stretches back to 1996, has won his past five bouts on points, most recently a rematch with Marcos Maidana in September.
British welterweight Amir Khan, who wants to fights Mayweather and also held talks with Pacquiao in January, said before the bout was announced that it is "not going to be what people expect".
He told the Daily Mail on Thursday: "Four years ago it would have been a massive fight because both of them were at their peak. It might have gone past that now.
"It will be a good fight for a couple of rounds. I think it will go the distance but that Mayweather will win by unanimous decision."
The fight also marks a rare joint agreement between Showtime Sports and HBO, the two American media companies, who traditionally work in opposition.
They have worked together before, teaming up in 2002 for the fight between then heavyweight champion Lennox Lewis, who was with HBO, and former champion Mike Tyson, who was with Showtime.
This time Showtime hold the rights to Mayweather while HBO have Pacquiao.
"This fight took about five or six years of negotiations, arguing, egos getting in the way before it finally came to fruition," explained Bob Arum, Pacquiao's promoter.
"It's the biggest fight by far in years in the sport of boxing, so I equate this to five or six years of Super Bowls all wrapped into one fight. It'll be the biggest boxing event of all time."
Negotiations for the fight first came to a halt five years ago but Stephen Espinoza, the executive vice-president of Showtime Sports, does not believe the advancing years of the two boxers will damage the event as a showpiece.
"Part of the myth and legend of this entire event is that is took so long to come to fruition," he said.
"Maybe a boxer that is just slightly past his prime provides a more entertaining, a more memorable fight than the guy whose reflexes are at their peak."
Floyd Mayweather shared the signed contract with Manny Pacquiao on social media.