Arnold Schwarzenegger -- Best Buddies Lifting Sesh ... With J.J. Watt

The Arnold Schwarzenegger and J.J. Watt bromance is HEATING UP -- with the two meeting up for a late-night gym sesh ... to blast out a few squats?!

Of course, the Houston Texans star struck up a friendship with Ahnold at the CMT Awards in Nashville last month -- and clearly, they've kept in touch.

So, with J.J. in L.A. -- it appears Arnold invited Watt back to his home gym ... where, according to Schwarzenegger, he challenged Watt to do 50 reps on his squat machine.

The result??

"He did 51. After dinner," Arnold said ... "Watch out, quarterbacks."

#BestBuds