Santa Rosa is a city in and the county seat of Sonoma County, United States, in the North Bay region of the Bay Area in California. Its population as of the 2020 census was 178,127. It is the largest city in California's Wine Country and Redwood Coast. It is the fifth most populous city in the Bay Area after San Jose, San Francisco, Oakland, and Fremont; and the 27th-most populous city in California.