The "Baby" singer reached out to UFC boss Dana White earlier this month to ask if he'd be willing to stage a bout between himself and the movie star, and White insists Justin isn't joking.
Now, Mayweather, who once trained Bieber, tells TMZ he's likely to be involved if the unlikely fight actually takes place.
"If they bring me in as a promoter, then I can talk about it," he said. "But until then, I gotta stay in my lane. Gotta stay in my business."
Meanwhile, Dana thinks the fight might happen too.
He has told the outlet, "I got a phone call from a couple of real guys who say they really do wanna do this fight and they believe Tom Cruise would do the fight. I told them that if that's true, and everybody involved really wants to do it, we can talk."
"I'd be an idiot not to make this fight... That would be the easiest fight to promote in the history of my career."
But it seems Justin may be having second thoughts about going toe to toe with Tom: "I'm pretty sure he'd whoop my a** in a fight. I'd have to get in good shape, I'm really skinny right now. I think he'd probably be out of my weight class. Because he's big, you know he's got that dad strength."