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.

The Vistula is the longest river in Poland and the ninth-longest in Europe, at 1,047 kilometres in length. Its drainage basin, extending into three other countries apart from Poland, covers 193,960 km2 (74,890 sq mi), of which 168,868 km2 (65,200 sq mi) is in Poland.