Sussex Police are investigating a serious collision on the A27 Eastbound between Fontwell and Tangmere.
A white Seat Leon hit a tree at around 11.30pm on Saturday.
The driver and a passenger suffered serious injuries and were taken to the Royal Sussex County Hospital, Brighton. A second passenger in the vehicle suffered critical injuries and was pronounced dead at the scene. Enquiries into identifying next of kin are ongoing.
The road remained closed until just before 10.10am the following day. Police are now searching for the driver of a vehicle who may have witnessed the A27 collision.
Officers believe the driver of another hatchback-style vehicle – black at the front and white at the back – may have witnessed the collision.
If you are the driver of this vehicle or recognise the description of the car, you are asked to contact police as you may hold vital information to aid the investigation.
Anyone else who witnessed the collision is asked to report online or call 101 quoting Operation Belford.