Black Mountain, Mountain summit in Santa Clara County, California, US

Black Mountain, also called Mount Day, is a peak in the Diablo Range of eastern Santa Clara County, about 10 miles (16 km) from Milpitas. Its northern half is oak woodland while the southern half is barren. At 1,170 m it is among the highest peaks in the area and can be seen from Sunnyvale, Cupertino, Mountain View and San Jose.

Black Mountain is also known as Mount Day, a name recorded for the peak in the Diablo Range of eastern Santa Clara County. The mountain rises to about 1,170 metres and lies roughly 10 miles (16 km) from Milpitas, east of Poverty Ridge and Arroyo Hondo. Its northern half is oak woodland while the southern half is barren.

Three significant waterways originate from Black Mountain: Stevens Creek, Adobe Creek, and Permanente Creek, each contributing to the local water infrastructure.

A hiking trail of about 2.5 miles leads from the Monte Bello parking area to the summit and requires moderate fitness. The path is clearly marked, though the climb is steady and demands good breathing capacity.

Broadcasting stations operated by Stanford University and the Federal Aviation Administration sit atop the summit. Many hikers pass by these structures without noticing their presence or purpose.