The incident occurred just after 9:30 p.m. on State Route 242 at Highway 4 in Concord.
The California Highway Patrol reports that a 20-year-old man was driving while the woman was in the front passenger seat and two children under the age of 12 were seated in the backseat.
As the Volkswagen Jetta approached Highway 4 from northbound SR-242, a "heavy object" came through the window, hitting the woman.
She died at the scene, according to highway patrol.
The cause of the incident and what led up to the object crashing through the window are now under investigation.
The CHP said it is unclear where the object came from or how it ended up in the Volkswagen.
Those with information on what happened are asked to contact the Contra Costa CHP at (925) 646-4980.