South East Coast Ambulance Service (SECAmb) said paramedics were called to A259 Bognor Road, shortly before midnight on Tuesday (January 4), after reports that a person had been involved in a collision with a vehicle.
A SECAmb spokesman added: "Ambulance crews attended and were joined at the scene by the air ambulance service.
"The pedestrian was assessed and treated before being taken to Royal Sussex County Hospital in Brighton by road with potentially serious injuries.
"The air ambulance team joined us in our vehicle to help treat the patient en-route to hospital."
Sussex Police confirmed the injuries were serious but not life threatening.
Sussex Roads Police officer Glen McArthur wrote about the incident on Twitter in the early hours of this morning (Wednesday).
PC McArthur praised the work of paramedics from SECAmb as well as volunteers from the Air Ambulance Charity Kent Surrey Sussex, commenting: "Great work, as usual."