Police said the collision involving a white VW Polo and a pedestrian occurred on the westbound carriageway, close to the slip road to the A285, at about 1.20am this morning (Sunday, November 5).
A section of the road was closed for several hours while emergency services worked at the scene, and Sussex Police have thanked the public are for their patience and understanding.
Police said the pedestrian – a 26-year-old local man – has been taken to hospital with life-threatening injuries.
Anyone who saw what happened or captured any footage of the incident is asked to email firstname.lastname@example.org quoting Operation Cog.
Police are particularly keen to hear from anyone who may have seen a man wearing a red hooded top and blue jeans walking in the area around the time of the collision.