Silicon Valley is a region in California.
San Mateo County, California . oʊ ] ; Spanish for `` Saint Matthew '' ) is a county located in the U.S. state of California . 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 . The region has expanded to include the southern half of the San Francisco Peninsula in San Mateo County , and southern portions of the East Bay in Alameda County .