Steve Whitmore, a spokesman for the Los Angeles County Sheriff's Department, said the incident allegedly occurred outside a Mexican restaurant Friday night.
He said the 54-year-old former James Bond star was not arrested or detained.
Messages left Tuesday for Brosnan's publicist, Jennifer Allen, were not immediately returned.
Whitmore identified the alleged victim as Robert Rosen, but declined to give more details because the investigation is ongoing.
Rosen could not be reached for comment.