The 34-year-old Pasadena man was the sole occupant of the car and was observed speeding and running a stop sign, according to Pasadena police Lt. Monica Cuellar.
Upon approaching the driver and asking for his registration, the detectives saw a handgun when the driver opened and reached into his glove compartment, Cuellar said.
The firearm discovered was an unserialized Smith & Wesson .40 caliber semi-automatic handgun, with a magazine containing 13 rounds.
Cuellar said the driver cooperated with the detectives and was later charged with firearm-related offenses.