Santa Cruz Island is located off the southwestern coast of Ventura, California, United States. It is the largest island in California and largest of the eight islands in the Channel Islands archipelago and Channel Islands National Park. Forming part of the northern group of the Channel Islands, Santa Cruz is 22 miles (35 km) long and 2 to 6 miles wide with an area of 61,764.6 acres (249.952 km2).

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.