Alvarez is 42-0-1. The 22-year-old fighter unified the 154-pounds titles April 20 in San Antonio with a unanimous victory over Austin Trout.
The fight will be contested at 152 pounds and offers a rare bout between two unbeaten fighters in their primes.
“Floyd has said from Day 1 that he wants to give fans the best fight out there and here it is,” Mayweather’s co-manager, Leonard Ellerbe, told the AP. “We have the two biggest stars in the sport and they’re fighting each other Sept. 14.”
The bout will be the second in Mayweather’s six-fight, 30-month contract with Showtime that could pay him more than $200 million.