Sunnyvale () is a city located in the Santa Clara Valley in northwestern Santa Clara County, California, United States.

Sunnyvale lies along the historic El Camino Real and Highway 101 and is bordered by portions of San Jose to the north, Moffett Federal Airfield and NASA Ames Research Center to the northwest, Mountain View to the northwest, Los Altos to the southwest, Cupertino to the south, and Santa Clara to the east.

Sunnyvale's population was 155,805 at the 2020 census, making it the second most populous city in the county (after San Jose) and the seventh most populous city in the San Francisco Bay Area.

As one of the major cities that make up California's high-tech area known as Silicon Valley, Sunnyvale is the birthplace of the video game industry, former location of Atari headquarters. Many technology companies are headquartered in Sunnyvale and many more operate there, including several aerospace/defense companies.

Sunnyvale was also the home to Onizuka Air Force Station, often referred to as "the Blue Cube" because of the color and shape of its windowless main building. The facility, previously known as Sunnyvale Air Force Station, was named for the deceased Space Shuttle Challenger astronaut Ellison Onizuka. It served as an artificial satellite control facility of the U.S. military until August 2010 and has since been decommissioned and demolished.

Sunnyvale is one of the few municipalities in California to have a single unified Department of Public Safety, where all personnel are trained as firefighters, police officers, and EMTs, so that they can respond to an emergency in any of the three roles.