The crash was reported just after 9:30 p.m. on the southbound 57 at Arrow Highway, according to the California Highway Patrol.
The incident unfolded after deputies with the San Dimas sheriff's station responded to a call about a woman who was reportedly flashing a gun near Bonita and Eucla avenues.
When they arrived, the woman ran onto the freeway and deputies requested help from the CHP. She was later found dead on the freeway after being struck by a vehicle.
The gun she was allegedly carrying has not been found.
Traffic was snarled as three southbound lanes of the freeway were shut down. Northbound lanes were not impacted by the crash.