The Wadden Sea (Dutch: Waddenzee, German: Wattenmeer, Low German: Wattensee or Waddenzee, Danish: Vadehavet, West Frisian: Waadsee, North Frisian: di Heef) is an intertidal zone in the southeastern part of the North Sea. It lies between the coast of northwestern continental Europe and the range of Frisian Islands, forming a shallow body of water with tidal flats and wetlands. It is rich in biological diversity. In 2009, the Dutch and German parts of the Wadden Sea were inscribed on UNESCO's World Heritage List and the Danish part was added in June 2014.

The Wadden Sea is one of the world's seas whose coastline has been most modified by humans, via systems of dikes and causeways on the mainland and low-lying coastal islands. The Wadden Sea stretches from Den Helder in the Netherlands in the northwest, past the great river estuaries of Germany to its northern boundary at Skallingen north of Esbjerg in Denmark along a total length of some 500 km and a total area of about 10,000 km². Within the Netherlands it is bounded from the IJsselmeer by the Afsluitdijk.