The robbery was reported at about 9:50 p.m. at The Sunnyvale Market at 1512 Sunnyvale Ave., according to police.
A suspect allegedly entered the store brandishing a handgun and ordered everyone to get on the floor before stealing money from the cash register, police said.
He then fled in a black, four-door BMW that had another passenger inside, according to police, who said that no one was injured during the robbery.
The suspect is described as a black man in his late 20s who is about 6 feet tall with an average build and bald head. He was last seen wearing dark clothing and a black hooded sweatshirt.
Anyone who can provide information about the robbery can contact the Walnut Creek Police Department at (925) 943-5844.