Fourth Squad detectives said the gunman -- wearing a black jacket and black hat -- took an unspecified amount of cash from a Domino's Pizza delivery man on Ruby Street at about 12:30 a.m.
The delivery man, 21, showed up at a residence with five pizzas, but the homeowner told him no food had been ordered, police said.
As he returned to his car, the delivery man was met by the gunman, who also took a cellphone and drove off in the victim's vehicle, police said.
No injuries were reported, police said.
Anyone with information can call Crime Stoppers at 800-244-8477. All calls will be kept confidential.