The crash happened shortly before 10 o'clock last night in the townland of Greenhills on the M9 southbound between the M7 merge and Junction 2 Kilcullen.
A man, in his mid-30s, was struck by a car and died at the scene.
His body has been removed to the Mortuary in Naas General Hospital.
The driver of the car, a man in his early 20s, was uninjured.
The road is closed southbound between junctions 1 and 2 to allow for an examination by Garda Forensic Collision Investigators, and local diversions are in place.
Gardaí are appealing to any motorists who may have travelled on the route or who may have camera footage to contact them at Naas Garda Station.