Canelo Alvarez will bid to become a four-weight world champion when he takes on Sergey Kovalev on November 2.
The Mexican has thrashed out terms with the light-heavyweight ruler and the pair will meet at the MGM Grand in Las Vegas.
Alvarez has previously won belts at light-welterweight, middleweight and super-middleweight.
He said: “It’s official. I am happy to announce that I will climb two (weight) categories to face one of the most powerful rivals in this division.
“November 2 will be one of the biggest challenges of my career. Let’s make history together, let’s go for four-time world champion.”
Alvarez unified the middleweight division last time out against Daniel Jacobs, a victory which followed triumphs over Rocky Fielding and Gennady Golovkin.
He had attempted to convince Kovalev to abandon his plans to fight Briton Anthony Yarde last month.
But the Russian put his belt on the line in his hometown, stopping Yarde in the penultimate round having himself survived a huge scare in the previous session.
He said after his victory: “I really want the fight with Alvarez, so if he has the same desire, then I’m only for that fight.
“[Alvarez’s team] played this cunning game, whereby they made an offer which was very hard to turn down when I was in training camp. But I promised Chelyabinsk, that I would be here. To fight in my home town was unbelievably cool.”