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.
