Mayweather, 41, a former five-weight world champion who retired from boxing with a perfect professional record of 50-0 last year, saw off an opponent 21 years his junior.
In a bout that was scheduled for three rounds just outside Tokyo, Nasukawa lasted only two minutes and 20 seconds before his corner threw in the towel.
Mayweather said in the build-up he expected to pocket nine million US dollars (£7.06m) from the fight, which headlined an event organised by Rizin Fighting Federation, a Japanese mixed martial arts organisation.
Mayweather had a 4kg (9lb) weight advantage over Nasukawa, who boasted an unbeaten record in 28 kickboxing bouts and four contests in MMA, with this fight having no bearing on either man's official record.
Nasukawa, though, reportedly faced a multi-million dollar fine if he attempted to kick out at Mayweather.
A grinning Mayweather started supremely confidently and dropped Nasukawa for the first time with a left hook with barely a minute gone.
A body shot followed by an uppercut saw Nasukawa slump to the canvas again moments later, with the southpaw bravely getting to his feet, albeit on unsteady legs.
A one-two from Mayweather precipitated the end of the contest, and the American said in quotes published by BBC Sport: "It was all about entertainment - we had a lot of fun. They wanted this to happen in Japan, so I said 'why not?'"
"I'm still undefeated. Tenshin is still undefeated. Tenshin is a true champion and a hell of a fighter.
"I want the fans around the world to support Tenshin, he's a great guy and a great champion."
