The robbery was reported at about 6:30 p.m. at the North Main Shell station at 2900 N. Main St., according to police.
A suspect had allegedly entered the cashier booth and pointed a handgun at the clerk, demanding money, police said.
The suspect fled the area on foot with an undisclosed amount of cash and several cartons of cigarettes.
A Pleasant Hill K-9 unit responded and tracked the suspect to a nearby Pleasant Hill Bart Station at 1365 Treat St., but was unable to locate him, according to police.
The suspect is described as a black man in his early 30s, about 5 feet 11 inches tall, clean shaven, with a medium build. He was last seen wearing a black jacket and baggy black pants, police said.
Anyone with information about the robbery should call the Walnut Creek Police Department at (925) 943-5844.