The crash happened around 4 a.m. near Topanga Canyon Boulevard, according to the California Highway Patrol's traffic log.
A motorcyclist, who has not been identified, was struck and killed.
Three other cars were involved in the collision, but additional details about what led up to it were not available.
AIR7 was over the scene when all westbound lanes were closed and drivers were being diverted off the freeway at Canoga Boulevard.
By 6 a.m., the far left lane was opened so drivers could get through but the investigation was still ongoing.
It's unclear when all lanes would open back up.