Charles Barkley -- I Tried to Save Johnny Manziel ... I Failed.

Charles Barkley says he confronted Johnny Manziel about his drinking and drug use during a chance encounter in Vegas -- but despite his best efforts, he just couldn't get through to the QB.

Barkley appeared on 97.5 "The Fanatic" and said he ran into Johnny in Sin City back in May.

"I said, 'Johnny, man you're screwing up your life, man. I'm not your dad, I'm not gonna try to lecture you. I'm just telling you I had a brother who passed away at a young age. Drugs and alcohol got him. It's gonna end like a bad movie.'"

"He said, 'Man I just like to have fun,' and I said, 'Johnny we all like to have fun, at some point it's out of control.'"

"I said, 'Man, don't screw up your career, and I'm worried about your life ... My mom, when my brother passed away, I thought she quit living after that happened. I felt bad for her, and I don't wanna feel bad for your mom and dad if something happens to you man, stop being selfish.'"

"He said, 'You know what, I'm gonna try and do better.'"

But the next day, Barkley quickly learned the message didn't get through after TMZ Sports reported he was involved in a fight in a nightclub hours after their heart-to-heart chat.

"My friends, they were riding me, 'Yeah, you're a good mentor, the guy gets arrested three hours after talking to you.'"

FYI, Manziel wasn't arrested -- but he was kicked out of the club.