If Jonathan Majors ever gets tired of being an A-list actor, weightlifting legend Ronnie Coleman tells TMZ Sports the dude could seriously be a pro bodybuilder!!

Coleman made the proclamation in a sit-down with us on Tuesday -- just a few days after he had put Majors through a grueling chest and arms workout in Colombia.

Coleman said he was so impressed with Majors' muscles during the gym session, he came away certain that if he coached Majors for around 6-to-12 months, the 35-year-old would be able to compete for hardware at real-deal physique shows.

"He is in awesome shape," Coleman said. "Awesome shape. He's in good enough shape now to get on stage and compete as a bodybuilder. And he's got an awesome physique. He trains pretty regularly."

Coleman, of course, knows a thing or two about what it takes to impress judges ... he's an eight-time Mr. Olympia who's widely considered one of the sport's biggest icons.

"He's got great shape," Coleman said of Majors. "He's in phenomenal condition. I was kind of shocked about his conditioning."

Majors, though, doesn't appear to be in line for any sort of career change just yet ... and Coleman clearly thinks that's a good idea, because he told us he recently got a sneak peek at the actor's new bodybuilding-themed flick, "Magazine Dreams," and he loved it.