Police said Danielle Lynn Flores, 24, assaulted the bartender at about 11:40 p.m. at the Gales Central Club at 106 Petaluma Blvd. North.
The bartender, whose name has not been released, suffered an injury to the side of her head, police said. The injury was not life-threatening.
Flores was arrested and booked into Sonoma County Jail on a charge of assault with a deadly weapon.