It happened just after 2:30 p.m. Wednesday in the parking lot of a bus rental company in Center Moriches.
Police say Doris Metz, of Mastic Beach, was walking across the lot when the truck, which had a a snow plow attached to the front, backed into her. She was pronounced dead at the scene.
Police say it was freezing rain at the time and the driver apparently did not see the woman.
The vehicle was impounded for a safety check and the investigation is continuing.