The Marin Independent Journal reports ( http://bit.ly/VZrLZj) the pickup crashed through a guardrail and went into the Nicasio Reservoir around 6 p.m. Monday.
Marin County deputies, firefighters and California Highway Patrol officers responded to the scene, with rescuers going into the water to try to save the man.
Marin County Fire Department Battalion Chief Mike Giannini say rescuers were able to pull the man from the water, but their resuscitation efforts failed.
The man was pronounced dead around 6:30 p.m. His name has not been released.
Divers were searching the reservoir Monday night to make sure no one else was in the water.
The area where the crash took place is in a rural area of Marin County, near the tiny community of Nicasio.
Information from: Marin Independent Journal, http://www.marinij.com