Joseph Baena's channeling his inner Mr. Universe ... with a weightlifting routine that would make his old man proud.
Arnold Schwarzenegger's son was at the gym Wednesday, and flaunted his muscles in a short vid of himself jacking up 120 lb. dumbbells on the incline press ... 4 times.
Baena doesn't quite have the bodybuilder bod yet ... but he's getting there. He's looking even more shredded than when we saw him with his smokin' hot GF around the Fourth of July.
And, speaking of Mr. Universe ... Arnold -- the winner of the 1969 competition -- JUST got reunited with his 1st place trophy a couple weeks ago after it went missing for decades.
Things are looking up for father and son.