Early reports indicated eight people were taken to hospitals, and police said the pedestrian, a woman, has serious injuries.
She was going west, against traffic, on the south side of Horseblock Road near Old Medford Avenue about 6:30 p.m. when she was hit by an eastbound car, police said.
That car then swerved into the westbound lane of Horseblock Road and hit two vehicles, a flatbed truck and the car behind the flatbed, police said.
The roads were closed while police investigated.
Suffolk police, who responded to the scene along with fire and emergency services from several communities, said the other victims had non-life-threatening injuries.
One of the vehicles was a flatbed truck with the name of a Farmingville business, M.I. Concrete Inc. The truck's flatbed portion had buckled and appeared to lean against the cabin.
At least one car had its front and back ends crumpled.
With Zachary R. Dowdy