Conor McGregor: Shirtless Showdown with Ronaldo

Two titans of sport ... not one single chest hair.

Cristiano Ronaldo and Conor McGregor came face-to-face in Vegas this weekend ... and immediately took their shirts off.

The soccer superstar hit up Conor's training gym in Vegas ... and even got in the Octagon with McGregor. Don't worry, Conor didn't touch his pretty little face.

After the meeting, the two decided to catch a show ... hitting up the Jennifer Lopez concert nearby.

BFFs?