Sonoma County is a county located in the U.S. state of California. As of the 2020 United States Census, its population was 488,863. Its seat of government and largest city is Santa Rosa. Sonoma County comprises the Santa Rosa-Petaluma Metropolitan Statistical Area, which is part of the San Jose-San Francisco-Oakland, CA Combined Statistical Area. It is the northernmost county in the nine-county San Francisco Bay Area region.