At about 6:15 p.m., a masked man entered a Quiznos located at 604 Santa Cruz Ave.
The suspect flashed his waistband at employees, showing he was armed, before he took an undetermined amount of cash from the register, police said.
The masked man then corralled the employees into the restaurant's back room and fled out the front door of the empty sub shop.
No injuries were reported, police said.
According to witness accounts provided to police, the suspect entered a dark-colored Ford Mustang that was seen exiting the parking lot and traveling southbound on El Camino Real.
The suspect is described as a white man standing 5 feet 9 inches tall and weighing 165 to 175 pounds. He was last seen with a large-caliber firearm. He was wearing a white shirt and a mask that covered his face and head.
Police ask anyone with information about the robbery to contact investigators at (650) 330-6300.