The California Highway Patrol says it happened just before 4 am Wednesday on the frontage road south of Cressy Way. That's between Atwater and Livingston.
Officers say a big rig driver thought he hit something. He pulled over, checked his cameras and realized he actually ran into a person.
The big rig driver called 911.
Authorities found the man with major injuries.
He was airlifted to a hospital in Modesto.
Officers say drugs or alcohol do not appear to be factors in the crash.