The American is taking on a Japanese kickboxer but the rules of the ring have not yet been agreed.
"I want to give the people what they want — blood, sweat and tears," Mayweather told a news conference in Tokyo on Monday, November 5.
Mayweather, now 41, went back into retirement last year with a record of 50-0 after dispatching Irish motormouth Conor McGregor in a huge moneyspinning fight in Las Vegas last year.
The American is said to be worth between US$700 million and US$1 billion but he seems to be ready for another huge payday — although it is not clear yet what his fee will be for the fight.
Old Enough to be Opponent's Father
His opponent on December 31 in Saitama, Japan, will be Tenshin Nasukawa, who at 20, is less than half his age.
Although he has signed a deal with Japanese mixed martial arts promoters RIZIN to fight Nasukawa it is unclear whether the bout will be under the Marquess of Queensberry's rules or some amalgam of boxing and kickboxing.
Mayweather had hinted at another fight with the Filipino boxing legend Manny Pacquiao — who is in the UK to speak at the Oxford Union — but there was little appetite for a rematch after the American won the first fight in 2015 so comprehensively.
There have also been rumors he was set to fight McGregor's UFC conqueror — Khabib Nurmagomedov — but the Daghestani fighter was unwilling to switch codes.
Teasing On Instagram
Mayweather posted a photograph on his Instagram feed on Sunday, showing him wearing kickboxing gloves.
"It's always been a goal of mine to go outside out of the US and display my talent. The world has never seen Mayweather compete live in Japan," said Mayweather.
"I want to continue to build my relationship with Money Team Tokyo. We've been making some huge fights happen in the US, but we look forward to taking the Mayweather Promotions banner and the TMT banner worldwide," he added.
Nasukawa boasts a 27-0 record in MMA but has never fought as a boxer, although last year he beat former IBF flyweight champion Amnat Ruenroeng in a kickboxing bout.
Mayweather is not a superstitious man so he will probably take no notice of the fact that one of the 20th century's most formidable boxers — "Iron" Mike Tyson — lost his unbeaten record in Japan in 1990 when he was knocked out by the unheralded James "Buster" Douglas.