The victim's body was found around 1:30 a.m. on Ridgeway Avenue in East Setauket.
Suffolk County police say the unidentified male was struck by a vehicle that was traveling westbound on Ridgeway Avenue.
The victim was pronounced dead at the scene by a physician assistant from the Office of the Suffolk County Medical Examiner.
The investigation is continuing.
Detectives are asking anyone with information about the crash to contact the Vehicular Crime Unit at 631-852-6555 or call anonymously to Crime Stoppers at 1-800-220-TIPS.