A victory for Mayweather would match the record of the late heavyweight champion Rocky Marciano, who retired in April 1956 with an unblemished record of 49-0.
"I'm ready to get back in the ring on September 12 and prove again to the whole world why I'm 'The Best Ever,'" said the 38-year-old Mayweather.
"I always bring my A-game and this fight against Andre Berto is no exception.
"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."
Haitian-American Berto, who has lost three of his last six fights, sounded a confident note about finding the range against the elusive Mayweather, rated one of the greatest defensive fighters of all-time.
"I'm coming to kick Floyd's ass on September 12," said Berto, 31, who has won his last two bouts.
"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. Somebody is getting knocked out and it won't be me."
The match will be the sixth and final fight of a record-breaking deal between Mayweather and Showtime Networks, leading to speculation it could be the farewell match for the American.
A world champion in five divisions, Mayweather made a reported $220 million from his unanimous decision over Pacquiao in what the most lucrative fight ever staged generating close to $600 million in revenue.
Two other world championship fights are also planned for the pay-per-view telecast.
Roman "Rocky" Martinez (29-2-2) will risk his WBO junior lightweight title in a rematch against the boxer he dethroned, four-time world champion Orlando "Siri" Salido (42-13-2).
Badou Jack "The Ripper" (19-1-1) will make the first defence of his WBC super middleweight title against mandatory challenger "Saint" George Groves (21-2, 16 KOs).
(Reporting by Larry Fine in New York; Editing by Steve Keating)
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