No one was injured in the holdup, which happened about 8:30 p.m. at the Citgo gas station convenience store, 2720 Main St.
The robber, who was wearing dark clothes and had a black gun, ran in a southeasterly direction through the parking lot toward Forest Street, police said.
Anyone who saw anything out of the ordinary at that time is asked to call Det. Matthew Seguin at 860-258-2043.