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The San Francisco Bay Area, commonly known as the Bay Area, is a populated region that surrounds the San Francisco and San Pablo estuaries in Northern California, United States. The region encompasses the major cities and metropolitan areas of San Francisco, Oakland, and San Jose, along with smaller urban and rural areas.[3] The Bay Area's nine counties are Alameda, Contra Costa, Marin, Napa, San Francisco, San Mateo, Santa Clara, Solano, and Sonoma.[3][4] Home to approximately 7.44 million people,[1] the nine-county Bay Area contains many cities, towns, airports, and associated regional, state, and national parks, connected by a network of roads, highways, railroads, bridges, tunnels and commuter rail. The combined urban area of San Francisco and San Jose is the second-largest in California (after the Greater Los Angeles area), the fifth-largest in the United States, and the 56th-largest urban area in the world.

The United States Office of Management and Budget (OMB) does not use the nine-county definition of the San Francisco Bay Area. The OMB has designated a more extensive 12-county Combined Statistical Area (CSA) titled the San Jose-San Francisco-Oakland, CA Combined Statistical Area[5] which also includes the three counties of San Joaquin, Santa Cruz, and San Benito that do not border San Francisco Bay, but are economically tied to the nine counties that do.

The San Francisco Bay Area is known for its natural beauty, liberal politics, entrepreneurship, and diversity.[6][7] The area has high incomes, performing above the state median household income in the 2010 census;[8] it includes the five highest California counties by per capita income and two of the top 25 wealthiest counties in the United States. Based on a 2013 population report from the California Department of Finance, the Bay Area is the only region in California where the rate of people migrating in from other areas in the United States is greater than the rate of those leaving the region, led by Alameda and Contra Costa counties