It happened on Middle Country Road in Smithtown around 3:30 a.m.
23-year-old Krystle Rivera was driving a 1988 Mercedes Benz when she apparently lost control of the car, which struck a tree and flipped over.
19-year-old Jonathan Havlicek of Mount Sinai was pronounced dead at the scene.
Rivera and two other passengers were taken to Stony Brook University Medical Center with non-life threatening injuries.
The vehicle was impounded for a safety check and the investigation is continuing.
Detectives are asking anyone with information to call the Fourth Squad at 631-854-8452 or anonymously to Crime Stoppers at 1-800-220-TIPS.
All calls will be kept confidential.