Saul 'Canelo' Alvarez will step back down to super-middleweight to defend his status as undisputed champion against Gennady Golovkin in a trilogy fight on 17 September.
Alvarez (57-2-2, 39KOs) remains the WBC, WBA, IBF and WBO champion at 168lbs despite being outpointed by Russia’s Dmitry Bivol earlier this month when venturing up to light-heavyweight.
The Mexican has decided against an immediate rematch and will instead look to settle a long-running feud against Golovkin, the WBA and IBF middleweight champion who will be moving up a division.
Many thought Golovkin won their opening bout in September 2017 only for a draw to be declared, while Alvarez claimed a narrow points decision 12 months later, with both contests held at middleweight.
"I feel very happy and proud to be able to give the best fights and this fight won’t be an exception," said Alvarez.
The venue for the third fight will be announced "soon" according to promoters Matchroom Boxing although Las Vegas is the early favourite, with the city’s T-Mobile Arena staging the first two showdowns.
"The Canelo v GGG trilogy is the biggest fight in boxing and I am delighted to get this made for September 17," said promoter Eddie Hearn. "These are two men that bitterly dislike each other and want to end this incredible series with a blistering KO.
"I truly believe this will be the most thrilling fight between these two great champions and it will be fireworks from the first bell to the last man standing."
The heavy-handed Golovkin (42-1-1, 37KOs), who at 40 is nine years older than his next opponent, has won all four fights since suffering the only defeat of his professional career against Alvarez.
Golovkin, who stopped Japan’s Ryota Murata inside nine rounds last month to become unified world middleweight champion again, said simply: "I hope to see you on September 17."