Justin Bieber has sworn to the high heavens that he never spit on his neighbor in a heated argument, but witnesses beg to differ -- and that could land the Biebs in big trouble.

Theoretically, anyway.

When the singer's neighbor approached to confront him about his dangerous driving, Bieber reportedly said, "Get the f--- out of here," spit in the guy's face, then chirped, "I'm gonna f---ing kill you."

Now TMZ reports that construction workers watching the scene from across the street confirm that Biebs indeed spit in the guy's face, because who needs to do their jobs when a pop star is fighting a skinny dude?

What's more, Bieber's neighbor says that the singer was so furious that his own bodyguard had to restrain him from lunging at the guy ... similarly to how he reacted to a smack-talking paparazzo in London.

While it was rumored that the neighbor may have had to a bone to pick with Bieber for a while, sources tell TMZ that the guy actually approached the guard shack of the gated community when Bieber first moved in and told them to protect his privacy.

If Bieber is indeed charged with sending his saliva into the face of another -- a legal form of battery -- he could be faced with a $1,000 fine (which is chump change for him anyway) and up to six months behind bars.

But lest we forget, it's California. By the time he finished mugging for his mugshot, he'd be out of there. The more likely scenario, if he were to be convicted, would be mandatory anger management classes and community service.

Yet one more thing the singing, dancing, smoking, sizzurp sipper has in common with his pal Chris Brown. Birds of a feather flock together, and in this case all the birds are angry.