Shortly before 1 p.m., the woman carried the explosive device into the station in the 11000 block of North Sepulveda Boulevard, LAPD Officer Mike Lopez said.
She told police the grenade belonged to her late brother, a war veteran, and she found it while cleaning out her garage, Lopez said.
The police bomb squad arrived, nearby homes and the station were evacuated, and the 405 Freeway's San Fernando Mission Boulevard ramps were shut down by the California Highway Patrol, Lopez said.
The bomb squad cleared the station about 3:15 p.m. No arrests were made and no one was injured.
Members of the squad took possession of the grenade. It's unclear if the explosive was detonated or found inert.
Police advised residents not to transport potentially explosive devices, especially to a public building.
"If something like that is found in your home, call us so we can come get it," Lopez said.