Roy Jones Jr. to Vitor Belfort, Gimme 6 Weeks & I'll Beat Your Ass

Boxing legend Roy Jones Jr. says he'll GLADLY accept Vitor Belfort's challenge -- telling TMZ Sports he's down to brawl with the ex-UFC champ ... but needs 6 weeks to prepare.

Just last week, 41-year-old Belfort -- the former UFC lightweight champ -- called out Roy during a media event for the ONE Championship MMA promotion.

"One fight I'd really love is me against Roy Jones Jr. Roy, wherever you are, I wanna come challenge you for a boxing match, but with this condition: I'm gonna be able to dirty box with you."

So, we hit up Roy to see if he was game -- and good news, the 50-year-old is ALL ABOUT THAT SMOKE!!!

"This is what I do for a living. Y'all must've forgot. That's who I am. I love a battle. I love a challenge. So, if that's what you want, that's what you get. Ain't like you gotta ask me twice."

RJJ hasn't fought in over a year (he picked up his 66th pro win in February '18) ... but says he doesn't need long to get into fighting shape.

"Well, I would like 8 weeks, cause I like to get right. I need 2 extra weeks just to make sure I can showboat. But, 6 weeks, I can whip you."

We asked Roy if he had a message for Vitor ... and did he ever.

"I got nothing but the utmost respect for you. You were a helluva MMA fighter. You one of the best of all time. But, when it comes to boxing, this is what I do. I'm the ruler. I'm the king of this thing. Ya understand me?"

"Since Muhammad Ali is not on Earth anymore, I don't think there's nothing ever on this Earth as bad as I am with these hands."

Be careful what you wish for ... you just might get it.