The Carquinez Strait is a narrow tidal strait in northern California. It is part of the tidal estuary of the Sacramento and the San Joaquin rivers as they drain into the San Francisco Bay. The strait connects Suisun Bay, which receives the waters of the combined rivers, with San Pablo Bay, a northern extension of the San Francisco Bay. It forms part of the border between Solano (to the north) and Contra Costa (to the south) counties, and is approximately 15 mi (25 km) north of Oakland.

The channel is navigable and is used for commercial and military shipping. The Napa River joins the strait, via the short Mare Island Strait, near its entrance into San Pablo Bay.