Suisun Bay (/səˈsuːn/ suh-SOON) is a shallow tidal estuary in northern California, USA. It lies at the confluence of the Sacramento and San Joaquin Rivers, forming the entrance to the Sacramento–San Joaquin River Delta, an inverted river delta. Suisun Marsh, the tidal marsh land to the north, is the largest marsh in California. Grizzly Bay forms a northern extension of Suisun Bay. The bay is directly north of Contra Costa County.