TMZ – Justin Bieber allegedly headbutted a guy at a pre-Grammy party last weekend, and cops are now investigating the incident as a possible crime.

Law enforcement sources tell us the alleged incident went down at 2 AM Saturday at L.A.’s Italian eatery, Serafina … where Justin’s producer pal, Poo Bear, was hosting a shindig.

Several eyewitnesses tell us Bieber was “play fighting” with Kyle Massey when he realized someone from the restaurant staff was recording the action. We’re told Justin asked the guy to stop recording and delete the video. The staffer apparently resisted and some partygoers started recording as well … that’s when the eyewitnesses say Justin lunged at the guy.

We’re told a restaurant employee — not the alleged victim — called the L.A. County Sheriff’s Dept and reported the alleged headbutt. When deputies arrived … Justin had already left. We got a shot of him on his way out.

We’re told the alleged victim doesn’t want to press charges — but the misdemeanor battery investigation is still going forward. 

Justin has not been in trouble with the law for more than a year, although a guy filed a police report against Justin Wednesday over a fight after an NBA finals game last June in Cleveland.

We reached out to Justin’s camp … so far no word back.

It was only a matter of time before the Biebs returned to his true form. We’ve seen glimpses of the old Bieber over the last year or so as he’s decided to whack off his locks, walk around Boston barefoot, throw down in a fistfight in Cleveland, announce he’s no longer taking pictures with fans, and delete his Instagram account (which is now back!). But nothing says I’m back to being my normal, bad boy self like delivering a good ol’ fashioned headbutt! I can’t say I totally blame the Biebs either. If somebody was filming you in what would be perceived by others in a negative light, and without any context, you’d be pissed as well. It’s a scummy move by that guy to take advantage of the Biebs solely to generate attention upon himself. Therefore, what better way to put a stop to that type of douchebaggery other than delivering a solid headbutt. You know what type of person delivers a headbutt? A person I don’t want to fuck with. Sure, people like you and I might’ve confronted this guy in a more civilized manner, but there’s a good chance this video still gets out there. Not unless you decide to put the fear of god in this guy with a headbutt, though! You stike that asshole with your cranium and he’s deleting that video real fast. Kudos to Bieber for standing up for himself in this situation.

We’ve all accepted your millions of apologies, Biebs. Get on back to being your bad boy self again!