Santa Rosa is a city in and the county seat of Sonoma CountyCaliforniaUnited States.[10] Its estimated 2014 population was 174,170.[8] Santa Rosa is the largest city in California's North CoastWine Country and the North Bay; the fifth most populous city in the San Francisco Bay Area after San JoseSan FranciscoOakland, and Fremont; and the 28th most populous city in California.