The man was in the front passenger seat of a car driven by an 80-year-old woman that struck the rear of another car at about 11:35 a.m. on Ralston Avenue, just east of Old County Road.
A chain-reaction crash resulted, according to police.
Emergency medical personnel transported the man to San Mateo Medical Center where he was pronounced dead, police said. His name has not yet been released.
No one else was hurt in the crash, but westbound Ralston Avenue was closed between Hiller Street and Old County Road for about three hours.
The accident remains under investigation.