San Bruno Mountain State Park is an open-space preserve in northern San Mateo County at the southern edge of San Francisco, centered on a four-mile ridge at the northern end of the Santa Cruz Mountains. Unusually close to dense urban development, it borders several cities yet remains a prominent natural landmark and a rare undeveloped island on the Peninsula. Trails climb to the summit and offer broad views across San Francisco and the Bay Area, while the mountain’s exposed slopes give it a distinctive presence. The park also protects sensitive habitat for rare and endangered plants and butterflies, especially the Mission blue butterfly.