Union City is a city in Alameda County, California, United States in the San Francisco Bay Area. The population in the 2020 census was 70,143, up from 69,628 in the 2020 census. The population is estimated to be about 70.3K in 2025. It is located approximately 19 miles south of Oakland, 30 miles from San Francisco, and 20 miles north of San Jose. It was incorporated in 1959, combining the communities of Alvarado and Decoto. Alvarado is a California Historical Landmark.