The robbery was reported at about 3 a.m. at the 7-Eleven store at 2525 N. Main St., police said.
Police said a man entered the store, brandished a handgun, took an undisclosed amount of cash from the cashier and ran from the scene.
Officers searched the area but were unable to find the suspect.
He described as an 18- to 22-year-old man with light skin who is between 5 feet 7 inches and 5 feet 10 inches tall with a thin build. He was last seen wearing a gray jacket and gray beanie with a dark bandana covering his face, police said.
Anyone with additional information about the crime is asked to contact Walnut Creek police at (925) 943-5844.