Muir Beach is a census designated place (CDP), unincorporated community, and beach that is located northwest of San Francisco in western Marin County, California, United States. It is named for John Muir. The population was 310 at the 2010 census. The community itself flanks the northwest side of the beach.
Located about 2 miles (3 km) from the entrance to Muir Woods National Monument, the beach is about 1000 feet (305 m) long and 200 feet (61 m) wide, with coarse sand and several large boulders. Redwood Creek empties into the beach. There is a parking lot at the beach, which is accessible via a footbridge.
The beach was formerly called Big Lagoon after a freshwater lagoon that was located where the parking lot is now. Damage from 20th century dairy farms interfered with the flow of the creek and the lagoon.