The fire, dubbed the Briggs Fire, broke out near Highway 126 and Briggs Road at approximately 1:39 a.m.
By around 7 a.m., the blaze burned 75-100 acres, the Ventura County Fire Department said. The fire, located in the hills above the river bottom south of the 126 Freeway, was not threatening any homes.
Helicopters arrived on scene and conducted water drops on the fire.
No injuries were reported. The cause of the blaze was not immediately known.