The suspect, believed to be a man in his early 20s, entered the store at 1888 Contra Costa Blvd. shortly before 2 p.m., police said.
He then brandished a handgun and demanded cash from a teller, according to police.
The suspect fled the store on foot and was last seen running into a parking lot near Washington Mutual and Safeway Inc., police seen.
The suspect is described as a black man about 5 feet 9 inches tall with a medium build, medium skin tone and short hair, and was last seen wearing a red bandana, red shirt and baggy blue jeans, police said.
On Oct. 23, two suspects, one armed with a handgun, entered the same MetroPCS store and demanded cash and merchandise from a teller, police said.
Those suspects also fled the scene on foot and were described as two black men, one in his early 20s, about 6 feet tall and wearing black clothing. The other was reportedly in his late teens, about 5 feet 4 inches tall and wearing black clothing as well.
Anyone with information about either robbery is asked to call police at (925) 288-4630.