The Browns star -- who was just named the starter over newly acquired Kareem Hunt -- loaded up 410 pounds on the bar and hit a perfect clean Wednesday.

If you're unfamiliar ... the lift is a power clean that's REALLY difficult -- even pro lifters struggle with weights over 400 pounds in the move.

But, Chubb didn't even react like it was a feat ... calmly poppin' the bar off his shoulders after he hit the perfect rep.

Sure seems like more than a coincidence that Chubb decided to post the vid Wednesday ... 'cause his coach, Freddie Kitchens, just told reporters Chubb is still the team's starting RB over Hunt.

Remember, the team just signed Kareem earlier this month ... and some wondered how soon it would be for the ex-Chiefs superstar to take over as the lead back.

But, seems it won't be that easy ... especially if Chubb keeps hitting the iron like this!!!

Nick -- one of Cleveland's two 2nd-round picks last year -- was a stud for the Browns as a rookie this past season, running for 996 yards and 8 TDs in only nine starts.

While Chubb's in the gym, his quarterback, Baker Mayfield, is taking a well-deserved vacation in Turks and Caicos.

Baker and fiancee Emily Wilkinson are relaxing by the beach with fellow NFL stars Saquon Barkley and Sterling Shepard, who also brought their significant others.

By the way, Sterling is married to Victoria's Secret model Chanel Iman ... and yeah, she looks goooooood.