A world champion boxer is the latest fighter to call out Jake Paul ... and the dude's so serious about making it happen, he got the YouTuber's name tattooed on himself!!
The man calling out Jake is Mairis Briedis -- he's a 28-1 (20 KOs) Latvian pro who is a 3-time cruiserweight champ and the current #6 pound-for-pound boxer, according to BoxRec.
36-year-old Briedis showed how badly he wants the bout by revealing the ink job on his left thigh on Monday ... saying, "Hey Jake, I got something for you.