Deputies responded to a battery report about 2 a.m. at Serafina Sunset restaurant in the 8700 block of Sunset Boulevard, according to sheriff's officials.
When they arrived, a witness told deputies that he saw Bieber fighting with two bartenders in two separate bouts, the Sheriff's Department said in a statement.
The witness told deputies he broke up the fights, authorities said. The witness then asked the 22-year-old Canadian performer to leave the Italian restaurant, but he refused, according to the Sheriff's Department.
A few minutes later, the witness said, Bieber was arguing with a customer, whom he then head-butted, deputies said.
"Mr. Bieber's friends pulled him away from the victim, and Mr. Bieber then left the location," the Sheriff's Department said.
Deputies interviewed the customer, who refused medical treatment and declined to file a police report against the "Love Yourself" singer, authorities said. The bartenders left before deputies got to the restaurant. The witness told deputies they also didn't want to file a report.
"West Hollywood Sheriff's Station detectives are investigating the incident to gather information and evidence in case the victim is desirous at a later date," the Sheriff's Department said.
Caroline McBride, spokeswoman for Serafina Restaurant Group, said the restaurant's staff was not involved in the alleged incident.
"We know very little about the alleged incident at Serafina Sunset with Justin Bieber, but we can assure you that nobody from our staff was involved, nor did we call the police," she said in a statement. "We always have pleasant experiences with Mr. Bieber whenever he comes to Serafina."
She said Bieber has frequented Serafina restaurants in New York City and has been kind to fans waiting outside for selfies with him.
Bieber's representatives did not immediately respond to a request for comment.
This isn't the first time Bieber has been investigated by law enforcement.
In 2014, the Los Angeles city attorney's office declined to file misdemeanor charges against Bieber in connection with an accusation by a woman who claimed he tried to snatch her cellphone at a miniature golf course in Sherman Oaks.
That same year, Bieber pleaded no contest to vandalism after his neighbor's home in Calabasas was egged. He paid more than $80,000 in restitution.