1151 GMT November 24, 2017
The former five-weight world champion, 40, retired unbeaten in 2015 after 49 bouts, but will face McGregor, 28, in Las Vegas on August 26, BBC reported.
The light-middleweight bout could earn both men as much as $100million (£78.4million).
"They asked for this fight. I was in retirement, but they wanted me back and I'm back," said American Mayweather.
Talking to Black Sports Online, Mayweather added: "McGregor is a tough competitor. People all around the world demanded this fight so I had to give them what they wanted to see."
Mayweather has not fought since beating Andre Berto in September 2015 and, against McGregor, could improve his career record to 50-0.
The UFC lightweight champion has never had a professional boxing match and some have described the bout as a "farce".
"He's very happy, I'm very happy and I can't wait," added Mayweather, who would not disclose how much each fighter was set to earn.
"You're supposed to stand behind your man. Stand behind the fighter that you believe in."
'I can't get my head around it'
Former world champion Ricky Hatton has first-hand experience of fighting Mayweather after he was stopped by the American in the 10th round during their world welterweight bout in Las Vegas in 2007.
The Englishman, who says he is a fan of both sportsmen, expects a "12-round onslaught" by Mayweather, adding that he "can't get excited about this fight for one minute".
However, he believes boxing and UFC "have to take the blame" for a fight that has divided opinion.
"I can't get my head around why the best pound-for-pound fighter in the world and arguably the greatest of all time is fighting someone who has never had a boxing match in his life," Hatton said.
"Look at the people Floyd Mayweather has beaten - Saul Alvarez couldn't lay a glove on him, Oscar de la Hoya couldn't sort him out - even me, I couldn't sort him out.
"It's going to be great entertainment, that's the most complimentary thing I can say about it."