Police are seeking witnesses to a collision between a car and a pedestrian on the A259 Marine Drive, Rottingdean, 50 metres east of its junction with Nevill Road.
It happened at 10.15pm on Sunday (July 1) and involved a 29-year-old Hove man on foot and a Skoda Fabia car travelling east, driven by a 75-year-old man from Saltdean.
The pedestrian sustained serious head and chest injuries and was taken by ambulance to the Royal Sussex County Hospital, Brighton.
The car driver and his passenger were unhurt.
Anyone who saw what happened, who may have dash camera footage of the incident or who may have other information is asked to contact police online or by phoning 101, quoting Operation Accord.