Paris Hilton In Alleged Lesbian Makeout Session -- Boyfriend River Viiperi Arrested

Paris Hilton MADE OUT WITH ANOTHER GIRL at a nightclub in Vegas early Monday morning ... and moments later, her new boyfriend was arrested for battery ... at least that's what the alleged victim is telling police.

TMZ has learned ... a male clubgoer filed a report with the Las Vegas Metro Police Dept. claiming his girlfriend was "making out" with Paris inside XS nightclub at the Encore hotel ... when Paris' new BF River Viiperi got upset over the hot girl-on-girl action.

The alleged victim claims Viiperi -- a 21-year-old male model -- punched him in the face sometime around 2:30 AM ... and injured him so badly, he needed medical attention.

The victim told hotel security he wanted to press charges against River ... and when cops arrived to the scene, Viiperi was arrested for misdemeanor battery. He was cited and released without ever going to the police station.

We're told Paris had already left the scene by the time cops arrived -- but she has NOT been accused of any wrongdoing.

Cops tell us the matter is currently under investigation.

We called Paris' rep for comment -- so far, no word back.