San Bruno Mountain is located in northern San Mateo County, California, with some slopes of the mountain crossing over into southern San Francisco. Most of the mountain lies within the  San Bruno Mountain State Park, a unique open-space island in the midst of the San Francisco Peninsula's urbanization. Next to the state park is the state San Bruno Mountain Ecological Reserve on the north slope. It is near the southern boundary of San Francisco, surrounded by the cities of South San Francisco, Daly City, Colma, and Brisbane.