The robbery happened at 9:41 p.m. as employees at the GameStop at 552 Contra Costa Blvd. were closing up for the night.
A man entered the store, pointed a silver revolver at clerks and ordered one of them to open the registers and put all the money in a bag he had brought with him, police said.
The man then remained in the store for about half an hour while he forced a clerk to try to open a safe, but the clerk wasn't able to open it, police said.
The man finally left the store with an undisclosed amount of cash.
He was described as a black man about 35 years old. He was about 5 feet 11 inches tall and weighed about 180 pounds.
He was wearing a tan hat with a bill that was not a baseball cap, a dark nylon jacket with a short collar over a black v-neck sweater, baggy blue jeans and black and white tennis shoes.
Anyone with information about the case is asked to call the Pleasant Hill Police Department at (925) 288-4630.