12:46 PM PT -- Mike Tyson has already emerged as a big betting favorite, according to BetOnline.ag.
The massive sports gambling site tells us ... Tyson is opening up as the -350 favorite -- which means gamblers would need to bet $350 on Tyson to win a $100 profit.
Roy is the +275 underdog -- which means a $100 bet pays $275.
11:25 AM PT -- As if this fight couldn't get any crazier ... we now have an undercard -- ex-NBA star Nate Robinson and YouTuber/boxer Jake Paul!!!
Remember, we spoke with Nate back in May, who called out the social media millionaire, saying, "I'm putting my everything on the line for (Jake). Let's get it done. It's that simple."
Robinson released a statement to The Athletic's Shams Charania about the fight getting done, saying, "I wanted to show that I'm a world class athlete, I played college football, played in the NBA for 11 years, and I'm excited for this venture into the sport of Boxing. Jake Paul, I want all the smoke.
Paul added ... "Make no mistake, on September 12th, Nate will meet the canvas early."
10:36 AM PT -- We just spoke with Mike Tyson and asked him the question everyone wants to know - is this a real-deal fight or more of a goofy, fun show?
Tyson insists he will be going FULL FORCE -- telling "TMZ Live," "Anybody who knows me, I don't know how to [take my foot off the gas]. I just know one way of fighting and that's just what it is."
In other words, Tyson says it's a REAL FIGHT.
Tyson also insists he's not doing the fight to get rich -- his cut is all going to charity because he's dedicated his life to helping people.
The boxing legend admits he's been through hell with cocaine, booze and weight issues -- and is now a completely different person, focused on health and philanthropy.
We also spoke with Roy Jones Jr. who's very excited for the fight -- telling us he's been ready for this for years!
Roy knows Tyson will be head-hunting from the opening bell -- but insists he's got a HUGE conditioning advantage over Mike.
"If he don’t kill quick, he's got problems on his hands," Roy tells TMZ Sports ... "If he don’t get it quick, his ass belongs to me!"
"I know what it feels to go 8 rounds, he done forgo -- y'all must’ve forgot -- you heard me??!"
We're told the fight will take place in California -- and will be sanctioned by the California State Athletic Commission.
The event will include an undercard and live musical performances from top artists -- and since Triller is backed by big stars like Snoop, Lil Wayne, Future, The Weeknd, Pitbull, Marshmello and more -- the entertainment should be pretty solid!
FYI, Triller is a rapidly growing social media platform that's gunning to take over the TikTok crowd -- and while it's previously focused on music (including partnerships with major labels) it's now making a concentrated foray into live events including sports.
Tyson has been talking about a comeback for months and got some massive offers -- so, we're guessing Triller shelled out some big bucks to land the rights to his first comeback fight.
There's more ... we've also learned Triller locked up the rights to a 10-part docuseries featuring Mike and Roy preparing for their fight ... with 2 episodes dropping every week until fight night, streaming on the platform.
As for the combatants, RJJ is 51 -- and held titles in 4 different weight classes ... middleweight, super middleweight, light heavyweight and heavyweight.
Roy most recently fought in Feb. 2018 -- but he's been training constantly and feels he can compete with Tyson.
Of course, Tyson hasn't had a pro fight since June 2005 ... and battled substance abuse and weight issues after he stepped away.
But now, Tyson says he's made health and fitness a priority again -- and wants to get back into the ring to help raise money for charity.
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Tyson had previously told us his sights were set on Evander Holyfield in a comeback fight ... and we're told that's still a possibility down the road -- but first up is Roy.