Silicon Valley is a county.
California ( [ kælᵻˈfɔːrnjə , _ - ni.ə ] , ) is the most populous state in the United States and the third most extensive by area . Santa Clara Valley . Most of the Santa Clara Valley is in Santa Clara County , including its county seat , San Jose . Silicon Valley is a nickname for the southern portion of the San Francisco Bay Area , in the northern part of the U.S. state of California . The `` valley '' in its name refers to the Santa Clara Valley in Santa Clara County , which includes the city of San Jose and surrounding cities and towns , where the region has been traditionally centered .