The westbound 118 was fully closed off around 10 p.m. while deputies searched for bullet casings, creating lengthy backups for late-night drivers.
Officials said the incident began around 8 p.m. when an off-duty Los Angeles sheriff's deputy in a patrol car noticed a muzzle flash from a car ahead of him on the westbound 118.
He requested backup and officers pulled over the car off the freeway in the 800 block of Gloria Avenue in North Hills. They found two men and two women inside.
Officials said one of the passengers admitted throwing the gun out of the car when they realized a patrol car was following them.
A male passenger was arrested for allegedly firing the gun into the brush at the side of the freeway. A woman was arrested on an outstanding warrant and the vehicle was impounded.
The other two people were released.
The 118 was reopened by about 11:30 p.m.