Emergency services were called to the junction of Eastdene Road and the B1285 on Tuesday (January 14) after a crash just before 1pm.
An air ambulance landed at the scene while several other ambulance vehicles and police cars blocked off the road to motorists and pedestrians.
Durham Constabulary said the pedestrian suffered serious injuries in the crash, and was taken to the Royal Victoria Infirmary, in Newcastle, by road ambulance with a doctor on board.
The force added that a driver has also been arrested on suspicion of causing serious injury by dangerous driving.
A spokesperson for North East Ambulance Service said: “We were called to a road traffic incident at the junction of Eastdene Road and the B1285, Seaham shortly before 12.45 this afternoon.
“We dispatched a rapid response paramedic, a clinical team leader and a double crewed ambulance. We were also supported by Great North Air Ambulance Service.
“One patient was transported to the Royal Victoria Infirmary, by road with a doctor on board.”
The air ambulance service said: “Our team have been activated at 12.52 to an incident near Seaham.
“The team arrived on scene at 13.06 and are currently working with the North East Ambulance Service to treat a patient.”
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Durham Constabulary said: “Police were called at 12.45pm this afternoon (January 14) to reports of a collision involving a pedestrian on Evesham Road, Seaham.
“The pedestrian has been taken to the Royal Victoria Infirmary Hospital in Newcastle with serious injuries.
“The driver has been arrested on suspicion of causing serious injury by dangerous driving.”