Topanga is an unincorporated area in western Los Angeles County, California, USA. It is located in the Santa Monica Mountains. Occupying Topanga Canyon, it is often referred to by that name. Topanga is 12,748 acres (52 km2) in size, and is bounded on three sides by State Park or conservancy lands, and on the south by the Pacific Ocean and a small strip of Malibu, which is the main community to the west. On the east is Pacific Palisades. Topanga has a population of 5,441 as of 2000. The ZIP code is 90290 and the area code is primarily 310, with 818 only at the north end of the canyon. It is in the 3rd County Supervisorial district.
Topanga Creek drains Topanga Canyon and is the third largest watershed entering the Santa Monica Bay.  The creek is one of the few remaining undammed waterways in the area, and is a spawning ground for steelhead trout. The area typically receives about 22″ of rain annually. Topanga State Beach lies on the coast at the outlet of Topanga Creek. Topanga Canyon Boulevard, State Route 27, is the principal thoroughfare, connecting the Ventura Freeway (US 101) with Pacific Coast Highway (SR 1). The southern portion of the boulevard largely follows Topanga Creek. North of the Old Topanga Canyon Road intersection, the boulevard traverses the Santa Monica Mountains.
Topanga Canyon contains lands of both Topanga State Park, which is the largest park in the Santa Monica Mountains, and the Santa Monica Mountains Conservancy. It is part of the Santa Monica Mountains National Recreation Area.