Suisun Bay (pronounced "suh-soon", /səˈsuːn/) is a shallow tidal estuary at 38°04′N 122°04′W / 38.07°N 122.07°W / 38.07; -122.07 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.

The bay was named in 1811, after the Suisunes, a Native American tribe of the area. The word originates with the Patwin.

On the west, Suisun Bay is drained by the Carquinez Strait, which connects to San Pablo Bay, a northern extension of San Francisco Bay. In addition to the major bridges at the Carquinez Strait, it is spanned in its center by the Benicia-Martinez Bridge and at its eastern end by the State Route 160 crossing (Antioch Bridge) between Antioch and Oakley.

It is the anchorage of the ghost or mothball fleet, a collection of U.S. Navy and merchant reserve ships. It was created in the period following World War II. Many ships were removed and sold for scrap in the 1990s, but 80 ships remain, including (until recently) the WWII battleship USS Iowa (BB-61). In 2010, plans were announced to remove the mothball fleet in stages, with final removal by 2017.