Chris Brown is in hot water ... again ... this time, for allegedly punching a Brazilian "model" in the face in the early morning hours of New Year's day.

Not surprisingly, Chris calls BS on Liziane Gutierrez's allegations -- he even made this bizarre video to explain he doesn't even know who the "dusty" ol' girl is!

Gutierrez claims she was partying in Chris' penthouse at the Palms Hotel in Vegas early Saturday morning when she tried to snap a pic of the singer with her cell phone. Phones were strictly prohibited in the suite, but someone Liziane got hers past security and when Chris saw it, he freaked and hit her in the face -- or so she says.

Las Vegas police confirm they are investigating the battery call and that Chris Brown is involved. No further information is available from the police or from Chris' reps at this time. Stay tuned ...