The man, who was seen wearing a mask and holding a handgun, entered a Cash n Go on Guerneville Road at about 6:50 p.m., according to police.
He left the building with an undisclosed amount of cash and fled the scene on a bicycle. Police were unable to find him.
The man is described as light skinned, in his 20s, 5 feet 7 inches tall and about 150 pounds. He was seen wearing a black mask, a dark-colored hooded sweatshirt and light colored pants. He was riding a blue bicycle, police said.
Anyone with information on this robbery is encouraged to contact Santa Rosa police at (707) 543-3639.