A California Highway Patrol officer on route patrol made the discovery around 5:15 a.m. on the northbound 605 just south of Carson Street, authorities said.
The officer found the man deceased in the driver seat of a Toyota 4Runner, with a gun beside him. The man, who has not been identified, suffered a gunshot wound in the chest.
The coroner was summoned to the scene. A homicide investigation is ongoing.
The CHP issued a SigAlert at 5:48 a.m., shutting down the two right lanes of the northbound freeway for the investigation.
The lanes were reopened just before 9 a.m.
City News Service contributed to this report.