Both combatants for Saturday night's megafight at T-Mobile Arena in Las Vegas came in under the 154-pound junior middleweight limit at Friday's weigh-ins.
Mayweather (49-0), a five-weight-class former world boxing champion, checked in at 149.5 pounds. McGregor, who is the only man ever to hold simultaneously hold two UFC weight classes, but 0-0 as a professional boxer, hit the scale at 153.
McGregor, of Dublin, Ireland, was the crowd favorite among the majority of the estimated 15,000 fans in attendance at Friday's event. When the duo posed for traditional post-weigh-in photos, McGregor launched a steady stream of invective at Mayweather, who took it all in with a stoic demeanor.
"He looks like (expletive)," said McGregor, who is the current UFC lightweight champion. "He looks blown out, full of water. That's the worst shape I've ever seen him. He looks blown out, out of shape."
Mayweather, a Las Vegas native, is competing for the first time since retiring after a Sept. 2015 victory over Andre Berto. He said that this will be his only time coming out of retirement, and he also believes he will finish McGregor's career.
"It won't go the distance," Mayweather said. "Mark my words. This will be Conor McGregor's last fight also."
While there is no official championship on the line for the bout, which is expected to challenge the record 4.6 million pay-per-view buys done by Mayweather's win over Manny Pacquiao in 2015, the World Boxing Council produced a special diamond-and-gold "Money" belt, which will be awarded to the winner.