The Lilac Fire was a fire that burned in northern San Diego County, California, United States, and the second-costliest one of multiple wildfires that erupted in Southern California in December 2017. The fire was first reported on December 7, 2017, burned 4,100 acres (1,659 ha), and destroyed 157 structures, before it was fully contained on December 16. The fire cost at least $8.9 million, including $5 million in firefighting expenses and property damage, and an additional $3.9 million in cleanup and erosion control costs. The fire threatened the communities of Bonsall, Oceanside, Vista, Fallbrook, and Camp Pendleton. During the fire, an estimated 10,000 residents were forced to evacuate, while a total of over 100,000 residents were forced to or advised to evacuate. On December 7, the Lilac Fire also cut the power to 20,000 people.

San Diego County, officially the County of San Diego, is a county in the southwestern corner of the U.S. state of California. As of the 2020 census, the population was 3,298,634, making it California's second-most populous county and the fifth-most populous in the United States. Its county seat is San Diego, the second-most populous city in California and the eighth-most populous city in the United States. It is the southwesternmost county in the 48 contiguous United States, and is a border county. It is also home to 18 Native American tribal reservations, the most of any county in the United States.