The East Bay is a commonly used, informal term for the lands on the eastern side of the San Francisco Bay, in the San Francisco Bay Area, in California, United States. Originally referring only to cities along the immediate northeast shore of the San Francisco Bay and the southeast shore of San Pablo Bay,^{[citation needed]} the region described as the "East Bay" has grown with the expansion of the Bay Area's population and infrastructure to include all of Alameda and Contra Costa counties, and even some farther outlying cities.

Institutions reflecting this regional identity include California State University, East Bay, the weekly East Bay Express newspaper, the AC Transit (Alameda/Contra Costa Transit) bus system, the Oakland East Bay Symphony the East Bay Regional Parks District, Eastshore State Park, the East Bay Municipal Utility District, the East Bay Green Corridor coalition, the East Bay Athletic League, The East Bay Monthly and the East Bay Bicycle Coalition. Historic institutions include the East Bay Electric Lines.