The undefeated boxer will take on Japanese kickboxing sensation Tenshin Nasukawa in what's being called a super bout challenge.
Also unclear following Monday's announcement was whether the fight will take place under the rules of boxing or mixed martial arts or some combination of the two.
"It wasn't easy to make this happen, but we told the people anything is possible and now we're here and we want to make sure we give the people in Tokyo what they want to see - blood, sweat and tears", Mayweather said during a press conference.
Floyd Mayweather says the form of his fight with Japanese kickboxer Tenshin Nasukawa is yet to be revealed.
Instead, Mayweather explained that the decision to fight in Japan was about expanding his global profile.
"I wanted to do something different".
"I wanted to display my skills outside the United States and be in a special fight".
'I spoke with my team and with RIZIN and we put it together.
"It's always been a goal of mine to go outside the USA and go into another arena and display talent as a professional because I did it as an amateur".
I would love to continue to work with RIZIN because RIZIN is an unbelievable company, and my company, we've been making some huge fights happen in the U.S., but we look forward to taking the Mayweather Promotions banner and the TMT banner worldwide. He has won all four of his MMA fights.
While boxing rules restrict bouts to the use of fists, MMA rules can incorporate kicks, knees and elbow strikes, as well as wrestling, joint locks and chokes. Tenshin Nasukawa will be present at this press conference, as will Floyd Mayweather, who recently arrived in Japan on his private jet.
"As far as the weight class and the rules, we will talk about that and will get that situated within the next couple of weeks", he said.
Rizin chairman Nobuyuki Sakakibara, who used to run the now-defunct Pride Fighting Championships promotional company, said they were still working out some of the details.
Sakakibara was clearly delighted with his coup.
In September, Mayweather said he planned to come out of retirement again and announced he would face Pacquiao in a rematch in December. It is believed that rules are still being negotiated and finalized between the parties and RIZIN. However, it is certainly a possibility that Mayweather vs. Nasukawa will be a boxing match.
Rules for the fight weren't announced.
"We do foolish things sometimes but I am not a damn fool".
RIZIN is clearly trying to make big moves, as you don't book "Money" for a fight without spending a massive amount of just that.