Floyd Mayweather Jr.

Floyd Mayweather Jr.

*Pound-for-pound king Floyd Mayweather Jr. and junior middleweight champion Saul “Canelo” Alvarez signed on Wednesday night to fight Sept. 14 in the main event of a Showtime pay-per-view event at the MGM Grand in Las Vegas in the biggest fight boxing has to offer, reports ESPN.

“I chose my opponent for September 14th and it’s Canelo Alvarez,” Mayweather tweeted to break the news. “I’m giving the fans what they want.”

Mayweather holds a world title at welterweight (147 pounds), but he also still owns a junior middleweight title (154), which he won by outpointing Miguel Cotto in May 2012. After that fight, Mayweather returned to welterweight and defended the title against Roberto Guerrero in a lopsided unanimous decision win on May 4 at the MGM Grand.

But he is moving back up in weight to face Alvarez (42-0-1, 30 KOs) with their belts on the line, although the fight will be contested at a catchweight of 152 pounds.

“It’s a done deal and Floyd is very excited to give the fans what they want,” Leonard Ellerbe, chief executive of Mayweather Promotions and one of Mayweather’s closest advisers, told ESPN.com. “This is the fight they wanted and this is what they are getting. It’s the biggest fight in all of boxing. It’s the biggest fight Floyd can possibly make.”

Mayweather, a five-division champion, and Filipino icon and eight-division champion Manny Pacquiao failed multiple times to come to terms on a super fight that loomed for years as one of the biggest in boxing history. While Alvarez is not at the level yet of Pacquiao in terms of fame and accomplishment, many view him as boxing’s top young star and have wanted to see the 36-year-old Mayweather (44-0, 26 KOs) take up the challenge of facing a younger, bigger, more powerful opponent.

Saul "Canelo" Alvarez

Saul “Canelo” Alvarez

The 22-year-old Alvarez, the 2010 ESPN.com prospect of the year, is Mexico’s most popular fighter and is coming off his biggest win. He claimed a unanimous decision against Austin Trout on April 27 to unify two 154-pound world titles before a crowd of about 40,000 at the sold-out Alamodome in San Antonio.

“I’m very excited. This is the fight I have wanted for a while,” Alvarez told ESPN.com from the Golden Boy Promotions offices in Los Angeles after signing his contract. “It’s been talked about for a while now, and now that it’s signed, sealed and delivered, I am very happy.

“I will be the first to beat Floyd.”