The incident, involving a Land Rover and a motorcycle, took place just before 4pm yesterday (December 4), near the village of Whatlington, in East Sussex.
Paramedics were called, alongside an air ambulance, to help the motorcyclist, who sustained serious injuries during the crash.
Police said the other vehicle, which has been described as a grey or dark-coloured Land Rover left the scene, having sustained damage to the front.
It is believed to have been travelling southbound on the A21 and to have left the area via Stream Lane toward Sedlescombe.
Officers from the Sussex Roads Policing Unit (RPU) are now appealing for information to locate the Land Rover and the driver.
They have also asked any vehicle repair garages in East Sussex to to contact them if a damaged Land Rover is brought in.
Anyone with information about the event is asked to email [email protected] or call 101 and quote serial 806 of 04/12.