It's possible ... with MMA legend Demetrious Johnson telling TMZ Sports he's down for a 3-round boxing match with Floyd Mayweather.

First off, DJ says Floyd is the one guy he would NEVER bet against in a fight -- "because he's just that good."

So, the respect is there.

But, now that Floyd's in the exhibition business -- where he's taking 3-round fights in Japan for roughly $10 mil-a-pop -- would Johnson be interested in testing his boxing skills against the best ever?

"Absolutely, I'll step in there and fight Floyd," 32-year-old Johnson said ... "3 rounds? Just boxing? I'll do that. It'll be fun! I feel like I'll do fine."

And, this isn't one of those hypothetical matchups either -- because as Johnson put it, "I think for Floyd Mayweather if it makes money sense, I think he would do anything."

Mayweather has already proven to be a big draw in Asia -- and with Johnson now fighting in Japan, his popularity is expected to skyrocket there too.

And, if DJ wins his March 31 fight against Yuya Wakamatsu at "ONE Championship: A New Era" -- it'll only help to create interest.