The fire started about 3:15 p.m. near Tustin Ranch and Portola Parkway, just west of State Route 261 near Citrus Ranch Park.
By 6 p.m., firefighters had the blaze fully contained. No injuries were reported.
Seventy-five firefighters were at the scene. Officials said the weather was cooperating and they were attacking the blaze with water drops.
The cause of the fire was unknown. Arson investigators were interviewing possible witnesses near the origin of the fire.