Warren County Coroner Jerry Borden said the dead man is 54-year-old Lanny Ross, of Oil City.
Ross was traveling on Route 27 in Pittsfield Township when the tree crashed onto his truck Monday about 2:50 p.m.
The highway was closed for several hours after the accident.
The coroner said it's unclear why the tree fell, though he believes recent rains may have softened the ground. Borden said the tree toppled over "roots and all" during clear weather.