Alum Rock Park, Urban park in Alum Rock district, San Jose, US

Alum Rock Park covers 720 acres (290 hectares) and contains oak woodlands, grasslands, and creek corridors with 13 miles (21 kilometers) of pathways winding through changing terrain. The routes pass through shaded canyons and open hillsides where views of the Santa Clara Valley appear between the trees.

Alum Rock Park became California's oldest municipal park when San Jose set it aside in 1872, on land public since the pueblo of 1777. Construction of a narrow-gauge railway from downtown San Jose began in 1891; by 1896 eight miles of track ran up Santa Clara and Alum Rock Avenues and along Penitencia Creek into the park, with passengers paying 25 cents. The line was electrified in 1901, washed out in the winter of 1911, and replaced in 1913 by the standard-gauge Peninsular Railway, which served the park until 11 July 1932.

The park's name comes from thenardite-bearing rocks near the entrance that were mistaken for alum around the turn of the 20th century. Penitencia Creek, properly Upper Penitencia Creek, took its name from a different creek to the north where monks, possibly from Mission San José, meditated by the water. Stonework grottos around the springs and old railroad support structures remain along the valley floor.

Visitors find picnic sites with grills, playgrounds, and restrooms at several points along the main road through the park. Early morning or late afternoon offers cooler conditions for hiking, especially during summer months.

White deposits near the entrance were misidentified as alum when they are actually different minerals, but the name stuck anyway. The spring that once drew bathers still flows at the same spot, though the bathhouses disappeared long ago.