The accident was reported at 8:18 a.m. near the intersection of Pacheco Boulevard and Brown Street.
The 56-year-old pedestrian, who was not in a crosswalk, was struck by the car, which stopped at the scene, police Lt. Jon Sylvia said.
The man was taken to John Muir Medical Center with injuries that are considered life-threatening, according to police.
The driver of the car cooperated with investigators at the scene.
The cause of the accident remains under investigation. Anyone who may have witnessed it is asked to call Martinez police at (925) 372-3436.