Shia LaBeouf -- Headbutts Guy in London Bar Brawl

Shia LaBeouf just headbutted a guy in a bar after what sounds like a nuclear argument over his girlfriend's mom ... and TMZ has the video.

At 6 seconds into the clip, you see Shia (wearing a white cap) headbutt a bald guy.

Shia screams at the guy, "What are you saying about my girl's mom? Are you f**king kidding me bro?"

You hear a girl -- presumably the girl Shia was with -- screaming his name in a plea for him to stop.

The fight went down at a pub called Hobgoblin in South London Thursday night -- in a pretty rough part of town.

Shia knows the pub well ... he was in a fight at the same joint in November, 2012. And his bar fight history doesn't end there ... he also got his ass whooped on the street outside a Vancouver bar in 2011.