The alleged carjacking occurred at about 1:40 a.m. in the Safeway parking lot in Washington Square, according to police.
The woman had just left the First Edition, a local bar located at 1420 E. Washington St., and had gone to the supermarket. As she got out of her vehicle, she reportedly saw a man she had seen at the bar.
The man allegedly stuck an object in the woman's back and told her it was a gun. He then reportedly forced her into the passenger side of her vehicle.
The man got in the driver's seat and headed southbound on Park Lane. Near the intersection of Park Lane and McGregor Avenue, the man reportedly pushed the woman out of the vehicle and fled the area in her car.
The victim described her attacker as a white man standing about 6 feet 1 inch tall and weighing about 225 to 230 pounds. He had a shaved head, numerous tattoos on his upper arms and was wearing a red baseball cap and red sweatshirt, police reported.
Police are asking anyone with information about the incident to call the Petaluma Police Department at (707) 778-4372.