Police are appealing for witnesses to the collision this morning between a HGV and a pedestrian on the eastbound A27 near Swandean Hospital at 9.35 am
The 29-year-old man was flown by air ambulance to Southampton Hospital neurological unit where he is in a critical condition. The lorry driver suffered shock.
The A27, its junction with the A24 and A280 were all closed for more than three hours for the emergency services to deal with the incident and for the air ambulance to land.
Anyone with information should call Sussex Police on 101 quoting Operation Daventry.