Deputies were called shortly after 11 p.m. to the 7-Eleven at 22760 Portola Drive.
A male believed to be in his 20s had pointed a black shotgun with wood stock - possibly a 12-gauge - at the store attendant and demanded money, the sheriff's office said.
The suspect allegedly took a small amount of money from the cash register. He also demanded additional, smaller-denomination bills in a box, the sheriff's office said.
The suspect then left through the front door and fled on foot.
He is described as 5 feet, 10 inches tall, and was dressed in black shoes, dark blue jeans, a black shirt, black Carhartt-style jacket with hood, black beanie hat, black scarf, black gloves and black sunglasses.