The attack was reported at about 2:45 a.m. on Broadway between Civic Lane and Fifth Avenue.
Officers responded and found the 48-year-old victim lying in the street and unconscious. He was transported to a hospital and his condition has not been released, police said.
Investigators determined that the man had been at the nearby Lariat Tavern at 1428 El Camino Real prior to the assault and are looking for possible security video in the area that may have captured the attack.
Anyone with information about the case is asked to call Belmont police at (650) 595-7400 or a tip line at (650) 598-3000.