Solano County (/səˈlɑːnoʊ/ ^ⓘ) is a county located in the U.S. state of California. As of the 2020 census, its population was 453,491. The county seat is Fairfield.

Solano County comprises the Vallejo–Fairfield metropolitan statistical area, which is also included in the San Jose–San Francisco–Oakland, combined statistical area. Solano County is the northeastern county in the nine-county San Francisco Bay Area region.

A portion of the South Campus at the University of California, Davis, is in Solano County.

Solano County was one of the original counties of California, created in 1850 at the time of statehood.

At the request of General Mariano Guadalupe Vallejo, the county was named for Chief Solano of the Suisun people, a Native American tribe of the region and Vallejo's close ally. Chief Solano at one time led the tribes between the Petaluma River and the Sacramento River. The chief was also called Sem-Yeto, which signifies "brave or fierce hand". The chief was given the Spanish name Francisco Solano during baptism at the Catholic Mission, and is named after the Spanish Franciscan missionary Father Francisco Solano.

Travis Air Force Base is located just east of Fairfield. Between 2017 and 2023, California Forever purchased over 50,000 acres of land in the county for an estimated $900 million to develop a new city.

Solano County is the easternmost county of the North Bay. As such, it is sometimes reported by news agencies as being in the East Bay. Additionally, a portion of the county extends into the Sacramento Valley, geographically.

According to the U.S. Census Bureau, the county has a total area of 906 square miles (2,350 km²), of which 84 square miles (220 km²), comprising 9.3%, are covered by water.