The Emerald Triangle is a collection of three counties in Northern California, consisting of Mendocino, Humboldt, and Trinity. It is the largest producer of marijuana in the United States.

Originally the majority of the marijuana was grown outside in the prime conditions of northern California, where there is enough sun and rain to grow redwoods, the tallest trees in the world. Years of police attention including the use of helicopters in the more rural areas have pushed much of the growing indoors. Because of the controlled conditions and the decades of refining strains such as "Trainwreck", Emerald Triangle marijuana is known nationwide for its quality.

This region is also called Behind the Redwood Curtain, because the two major highways that connect these three counties, U.S. Highway 101 and California State Highway 299, are narrow, winding and lined with tall redwood trees. This cuts the Emerald Triangle off from the rest of California.