Officers from Somerville Highway Patrol are investigating following a collision in Carrum Downs this morning.
Investigators have been told a 49-year-old man was standing on the Frankston Dandenong Road attempting to flag cars down about 12.50am.
A red Holden Commodore wagon travelling north on Frankston Dandenong Road, struck the pedestrian when he stepped out in front of the vehicle.
The pedestrian sustained serious injuries and was taken to hospital.
The driver of the vehicle, an 18-year-old Dandenong North man, stopped and assisted at the scene.
The investigation is ongoing and police are appealing to anyone who may have seen the man on the road prior to the collision or who has dashcam footage, to contact Crime Stoppers on 18000 333 000 or submit a confidential crime report at www.crimestoppersvic.com.au