The two hooked up at Cyborg's gym in Huntington Beach, CA on Thursday for a training session -- and anyone who's ever worked out with Cris before knows she doesn't mess around!

So, why are they training together? Halle is starring in "John Wick 3" and she's been doing heavy MMA training for the role ... so it's possible she was working with Cris to prep for a fight scene.

Or maybe she just caught the MMA bug after all of the Brazilian Jiu-Jitsu work she's been putting in for the part.

Either way, Halle had a great time -- saying, "From the red carpet to the ring - so inspired by your tenacity, Cris Cyborg - can't thank you enough for your hospitality."

Cyborg is an aspiring action movie star too -- she starred in "Fight Valley" back in 2016 and was tapped for the sequel.