Sonoma County is a county in the U.S. state of California. It is located north of the San Francisco Bay. It is the northernmost of the nine San Francisco Bay Area counties. Sonoma County is surrounded by 4 counties: Marin County to the south, Mendocino County to the north, Lake County to the northeast, and Napa County to the east. The county seat is in Santa Rosa.
Sonoma County was one of the original counties formed when California became a state in 1850.
The U.S. Census Bureau estimates that the population was 500,292 in 2014.