Sunnyvale (IPA: sʌniveil) is a city in Santa Clara County, California, United States. It is one of the major cities that make up the Silicon Valley. As of the 2000 census, the city population was 131,760.

The city is bordered by the San Francisco Bay to the north, Mountain View to the west, Santa Clara to the east, and Cupertino to the south. It lies along the historic El Camino Real and Highway 101.

Sunnyvale is headquarters to several high-tech companies such as Palm, Inc., AMD, Network Appliance, Sandisk, Yahoo!, and Ariba, Inc. Honeywell and Lockheed Martin also have offices in Sunnyvale. Sunnyvale is also the home to Onizuka Air Force Station (commonly called "the blue cube"), the only active military base in the San Francisco Bay Area. The base —which is named after Ellison Onizuka—is the United States armed forces primary artificial satellite control facility.

Sunnyvale is one of the few U.S. cities to have a single unified Department of Public Safety where all personnel are trained as firefighters and police officers and can respond to emergencies in either capacity.