LAS VEGAS — Floyd Mayweather Jr. will return to the ring for the first time since boxing’s richest fight ever, facing Andre Berto on Sept. 12 in what he has said will be his final fight.

The welterweight title bout doesn’t figure to be nearly as lucrative as Mayweather’s May 2 fight with Manny Pacquiao that earned him a reported $220 million. But it will still be sold on pay-per-view, even though Mayweather will be an overwhelming favorite to win for the 49th time in as many fights as a pro.

Berto, a 2004 Olympian, was once thought of as a rising star but has lost three of his last six fights. Two of those losses were to Robert Guerrero and Victor Ortiz, fighters that Mayweather beat easily.

“I always bring my A-game and this fight against Andre Berto is no exception,” Mayweather said in announcing the bout. “He’s a young, strong fighter who is hungry to take down the best. Forty-eight have tried before and on September 12, I’m going to make it 49.”

The fight is the last in a six-bout deal that Mayweather has with the Showtime network, and the boxer has said repeatedly he will retire when the deal is finished. The 38-year-old retired once before, but came back to fight after taking a short break.

“Floyd has been completely consistent and hasn’t wavered a bit in saying this is his last fight,” said Stephen Espinoza, executive vice president and general manager for Showtime Sports. “He’s still arguably the best fighter in the sport so it’s somewhat surprising for him to step away now but I’ve never heard him say anything other than this will be his last.”

Espinoza dismissed criticism that the fight looks like a mismatch, saying there were complications trying to do a deal with Britain’s Amir Khan and not a big pool of other fighters to choose from. He said Berto will be aggressive against Mayweather, something Pacquiao was not able or willing to be in their fight.

“He’s one of those fighters who never fails to entertain,” Espinoza said about Berto. “His biggest flaw is he’s probably too aggressive.”

Mayweather waited until the last possible moment to announce the fight, which will take place at the MGM Grand arena where he won a unanimous decision over Pacquiao. Mayweather and Berto will get together in Los Angeles on Thursday to promote the bout, which is less than six weeks away.

Berto (30-3) won his first 27 fights as a pro after losing in the first round of the Athens Olympics, winning a piece of the welterweight title along the way. But he was then beaten by Ortiz and would go on to lose back-to-back fights to Guerrero and Jesus Soto Karass.

Berto, who has rebounded to win his last two fights, said he will prove his worth in his biggest bout ever.

“Best believe that I plan to bring it to Floyd and I’m not concerned about what 48 other fighters have been unable to do,” he said. “Somebody is getting knocked out and it won’t be me.”

If Mayweather beats Berto he would improve to 49-0, matching the perfect record set by Rocky Marciano before he retired as heavyweight champion in 1956. There has been speculation Mayweather would fight at least once more to break the record, perhaps opening the MGM’s new arena on the Las Vegas Strip next spring.