The fight, which will be contested under "special rules", is slated on New Year's Eve at the Saitama Super Arena.
"It wasn't easy to make this happen", a subdued and polite Mayweather said.
The promoters said they are still working on the specific rule set and it is unknown whether it will be a boxing match, an MMA fight or a fight under different rules. "I want to give the people what they want - blood, sweat and tears".
Mayweather's record now stands at 49-0 after he beat Conor McGregor in Las Vegas a year ago.
Mayweather, who stopped McGregor in the 10th round past year after coming out of retirement for a $275 million pay day, had been flirting with the idea of a rematch with the Irishman or facing McGregor's recent MMA conqueror, the fearsome Russian Khabib Nurmagomedov.
"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", Mayweather added.
You get the feeling that he is driven largely by the desire to make more, though, and his latest cash-in will see him fight 20-year-old Tenshin Nasukawa (on the right in the below image), a 27-0 kickboxing and MMA sensation from Japan.
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That victory improved his boxing record to a flawless 50-0, but the format or rules for his meeting with the baby-faced Nasukawa were not specified by the Japanese mixed martial arts promoters RIZIN.
"I want to continue coming over here and bringing other fighters so my company can continue to grow and RIZIN can continue to grow".
Sources have confirmed to BJPENN.COM that Floyd Mayweather Jr.is set to appear at an imminent RIZIN Fighting Federation press conference. Are you excited to see it? I think when it comes to experience, I have more experience on my side but when it comes to youth, youth is on his side. "I'm so happy that Mr. Nasukawa accepted this offer in a flash".
"I just want to entertain".
"I can wrestle, I can wrestle little bit". I wanted to display my skills outside the USA and be in a special fight.
"Nobody has defeated my opponent in the past and I'd like to be the man to make history".
Ariel Helwani said tonight that the rules and weight still had to be worked out.