Police were called to the Palms Casino Resort on Saturday morning (local time), after an alleged altercation with Liziane Gutierrez at a private party.
Gutierrez said she was struck by Brown and that he had taken her cellphone, police said. Detectives tried to speak with Brown at his hotel room, but the singer left before police had arrived.
Police said they were investigating Brown as a suspect for misdemeanor theft and misdemeanor battery.
Brown took to Instagram to deny the allegations, in a since-deleted video where he described Gutierrez as "old and dusty".
"I don't know this old-looking b****," Brown said in the video.
"She probably came to my room and was too ugly to get in."
Nicole Perna, a publicist for Brown, issued a statement on Saturday night calling Gutierrez's allegations "unequivocally untrue".
The model "was escorted out of the private after party at the Palms Casino Resort for being disruptive and out of control," Perna said.
"Once she was in the hallway, while waiting for Chris Brown's security to bring out her phone, she had a total meltdown -throwing her purse to the ground and claiming that she 'could buy everyone in the hotel' - as witnessed by numerous people waiting to get into the party.
"The Palms Casino Resort security also saw her wild behavior via hotel security cameras and immediately came up to the hallway to escort her out of the hotel.
"Her claim that she had her phone in her possession inside the after party and was able to take a photo causing an altercation with Chris Brown is a complete fabrication," Perna said.
Gutierrez told TMZ she has snuck her phone into the private party without Brown's security team noticing. She said she called police a few hours after the alleged assault, but did not go to hospital.
The website ran a photo, appearing to show Gutierrez with a black eye.
Brown performed Friday at Drai's nightclub at The Cromwell Las Vegas, where he appears regularly.
Brown late last year cancelled an Australasian tour amid controversy over whether he should be granted a visa for Australia or New Zealand.
The singer seriously assaulted his then-girfriend, fellow singer Rihanna, in 2009.