Sierra Azul Open Space Preserve, Protected area in Santa Clara County, California.

Sierra Azul Open Space Preserve is a protected area spanning nearly 20,000 acres of natural landscape, including wooded canyons, chaparral hills, meadows, and mountain peaks. The terrain encompasses diverse ecosystems, with each zone supporting different plants and wildlife adapted to its conditions.

The preserve takes its name from the Sierra Azul, the "Blue Mountains" the Spanish colonizers called the half of the Santa Cruz Mountains south of today's Highway 17. Its highest summit, Mount Umunhum at 3,486 feet, later carried a Cold War radar station before the land became a public preserve managed by the Midpeninsula Regional Open Space District.

The name Sierra Azul, "Blue Mountains", was given by Spanish colonizers to the half of the Santa Cruz Mountains south of today's Highway 17. Mount Umunhum takes its name from the Amah Mutsun language, in which it refers to a hummingbird's resting place. The peak remains sacred to the Amah Mutsun Tribal Band, descendants of the region's first inhabitants.

The preserve has a network of trails that visitors can explore, accessible from several entry points where parking is available. Arriving early is recommended, especially on weekends, as the area becomes crowded during good weather.

Los Gatos Creek rises just south of Loma Prieta and loops around the preserve's western side before entering Lexington Reservoir, while Guadalupe Creek starts east of Mount Umunhum and feeds Guadalupe Reservoir. Both are headwater tributaries of the Guadalupe River, so rain falling inside the boundary reaches San Jose's water supply.