Bodybuilder Big Boy Guarantees Knockout Win Over Tito Ortiz, 'Retire His Ass!'

Bodybuilding star Big Boy -- who has millions of followers on social media -- is dead serious about boxing Tito Ortiz ... telling TMZ Sports he's gonna knock the ex-UFC legend into retirement!!!

The two sides have been jawing back-and-forth on the Internet recently ... with Ortiz claiming last month he didn't start the beef, but he's willing to settle their differences in the ring.

As for Big Boy, he insists it's Ortiz who started running his mouth ... and he tells us he actually has a lot of appreciation for what he's done throughout his career -- but that doesn't mean he doesn't wanna knock the dude's block off.

"I respect all fighters, 100%," Big Boy tells us. "He's a world champion, I've looked up to him and I respect him. But I think he's past his prime."

"I think I'm more explosive. I'm stronger. I got more game than him. Everything all in one. I just think it's his time, man. I'm gonna really retire his ass."

In fact, Big Boy says he's gonna make quick work of Tito ... guaranteeing a first-round knockout.

While Big Boy is mostly known for his iron-pumping, he says he's been hitting the mitts with huge names in fighting like former heavyweight champ Andy Ruiz in hopes of landing a fight ... and now, it sounds like he'll get his wish in 2023, 'cause Ortiz is on board.

"This guy wants smoke, let's light this s*** up, let's find a promoter, let's get it done," Tito says. "I'll get back in the gym. I'll start boxing. I know I could do some work and make it look easy."

Big Boy is so confident he'll come away with his hand raised ... he tells us he's already got his next opponent in mind -- Jake Paul.