The Vistula Lagoon is a brackish water lagoon on the Baltic Sea roughly 56 miles (90 km) long, 6 to 15 miles wide, and up to 17 feet (5 m) deep, separated from the Gdańsk Bay by the Vistula Spit.

A lagoon is a shallow body of water separated from a larger body of water by a narrow landform, such as reefs, barrier islands, barrier peninsulas, or isthmuses. Lagoons are commonly divided into coastal lagoons and atoll lagoons. They have also been identified as occurring on mixed-sand and gravel coastlines. There is an overlap between bodies of water classified as coastal lagoons and bodies of water classified as estuaries. Lagoons are common coastal features around many parts of the world.