The incident was on Old Country Road at 4:40 a.m. Thursday, police said. It was not clear how much cash was taken.
Police said the robber was armed with a silver handgun. They said no one was injured, but said the man ran north on Carlton Street following the early morning robbery.
The man is described as having a medium to heavy build and was wearing black sweatpants, a black hooded sweatshirt and a black mask. Police are asking anyone with information to call Crime Stoppers at 800-244-TIPS. All calls will remain confidential.