Saul 'Canelo' Alvarez will aim to become a four-weight world champion when he takes on Sergey Kovalev on November 2.
A third meeting between Gennady Golovkin and Canelo had been rumoured but negotiations fell through, leading him to instead move up to take on WBO champion Kovalev.
Canelo, whose last bout was a unification win over fellow middleweight Daniel Jacobs, described Kovalev in a tweet announcing the clash as "one of the most powerful rivals in this division".
"The second phase of my career is continuing just as we had planned," he said.
"And that's why we are continuing to make great fights to enter into the history books of boxing.
"That's also why I've decided to jump two weight classes against one of the most feared champions of recent years.
"Kovalev is a dangerous puncher, and he’s naturally the bigger man, but that's the kind of challenges and risks that I like to face."
Kovalev, 36, has been beaten in three of his past seven bouts but will not shirk the challenge of facing Canelo, with their clash to be staged in Las Vegas.
"In order to be the best you have to beat the best," Kovalev, who beat Anthony Yarde in Russia last month to retain the WBO strap, said.
"I have always tried to fight the toughest opponents in my division, but many have ducked me throughout my career.
"Canelo wanted to fight me; to step up to higher weight and challenge for my belt. I will be ready."