The East Bay is a subregion of the San Francisco Bay Area, California, United States and is comprised of Alameda and Contra Costa Counties. It lies on the eastern shores of the San Francisco Bay and San Pablo Bay and is generally considered to include not just the shorefront cities but the nearby inland valleys located on the east side of the Berkeley Hills.

San Francisco Bay's eastern shore was once known as "Contra Costa" (literally "opposite coast"), a name now used for and exclusively associated with Contra Costa County, which occupies the East Bay shoreline's northern quarter and extensive inland areas, amounting to about half of the land area, with the remainder in Alameda County.

Except for its mountains and hills which remain undeveloped and some farmland in eastern Contra Costa and Alameda Counties, the East Bay is highly urbanized. The East Bay shoreline is an urban corridor with several cities exceeding 100,000 residents, including, Oakland, Hayward, Fremont, Richmond, and Berkeley. In the inland valleys on the east side of the Berkeley Hills, the land is fairly well developed, but there are areas which are still rapidly growing, particularly on the eastern fringe of Contra Costa county and the Tri-Valley area. In the inland valleys, the housing stock tends to be more suburban, the population density less and the cities smaller. The only cities exceeding 100,000 residents in the inland valleys are Antioch and Concord.