Police said the assailant, armed with a knife, approached a 20-year-old man walking along North Park Avenue in Rockville Centre early Sunday and "demanded his stuff."
Nassau County police said investigators are searching for the robber who stole the victim's cellphone and wallet, containing credit cards and his New York State driver's license, before fleeing on foot.
The robbery took place near the intersection with Hillside Avenue at about 3:30 a.m.
The victim, whose identity was not released, was not injured during the robbery.
Police said the robber is believed to be in his late 20s and about 6-foot-2 with a stocky build and bald head. Police are asking anyone with information to call Crime Stoppers at 800-244-TIPS. All calls will remain confidential, police said.