San Clemente Island is the southernmost of the Channel Islands of California. It is owned and operated by the United States Navy, and is a part of Los Angeles County. It is administered by Naval Base Coronado. It is 21 miles (34 km) long and has 147.13 km2 (56.81 sq mi) of land. The 2018 census estimates 148 military and civilian personnel reside on the island. The city of San Clemente in Orange County, California is named after the island.

The Channel Islands are an eight-island archipelago located within the Southern California Bight in the Pacific Ocean, off the coast of California. They define the Santa Barbara Channel, which sits between the islands and the California mainland. The four Northern Channel Islands are part of the Transverse Ranges geologic province, and the four Southern Channel Islands are part of the Peninsular Ranges province. Five of the islands are within the Channel Islands National Park, and the waters surrounding these islands make up Channel Islands National Marine Sanctuary. The Nature Conservancy was instrumental in establishing the Channel Islands National Marine Sanctuary.