The 3-foot-long turtle was discovered Wednesday near Lands End Road in Orient Point.
The Riverhead Foundation for Marine Research and Preservation will perform a necropsy to determine what killed the female turtle.
It says loggerhead turtles are quite common in Long Island waters. It says 87 sea turtles washed ashore on Long Island last year; 38 were loggerheads. Not all were dead.
Foundation head Rob DiGiovanni tells Newsday that as water temperatures drop, turtles can become hypothermic. When they wash ashore sometimes they look like they're dead, but in many cases they might not be.
DiGiovanni says any sightings of marine life on the beach should be reported to 631-369-9829.