At about 9:20 a.m., a CHP officer on patrol saw a driver who appeared to be impaired heading east on state Highway 4 near Port Chicago Highway, and initiated an enforcement stop, Concord police said.
The driver exited onto southbound Port Chicago Highway and pulled over near Panoramic Drive.
When the officer went to talk to the man, who was alone in the car, the man shot himself, Dyer said.
He was transported to John Muir Medical Center in Concord, where he died from his injuries.
The CHP, Concord police and the Contra Costa County District Attorney's Office are investigating the case, as is protocol whenever there is an officer-involved fatality.
Southbound Port Chicago Highway was closed to traffic between Panoramic Drive and Arnold Industrial Way for about five hours during the investigation.
The off-ramp from eastbound state Highway 4 to Port Chicago Highway was also closed.
The area was reopened at about 2:30 p.m., police said.