Conor McGregor's next fight will be in the OCTAGON -- so says Dana White, who tells TMZ Sports the Irish superstar is 100% going back to the UFC to "do what we do."

We spoke with White minutes after Mayweather racked up victory #50 over Conor on Saturday -- and Dana said he couldn't be more proud of the 29-year-old fighting sensation. 

"That kid went out there tonight, 0-0 in boxing ... and went 10 rounds with the best to ever do it."

White says he's not sure who Conor will fight next in the UFC -- but he's certain there will be no follow-up boxing match for Conor. 

Dana's not worried about Conor going Hollywood either -- "I don't know about movies. He's a fighter, not a movie star."