The Wadden Sea (Wattenmeer in German, Waddenzee in Dutch, Wattensee in Low Saxon, Vadehavet in Danish) is the name for a body of water and its associated coastal wetlands lying between a section of the coast of northwestern continental Europe and the North Sea.

The Wadden Sea stretches from Den Helder in the Netherlands in the southwest, past the river estuaries of Germany to its northern boundary at Esbjerg in Denmark along a total length of some 500 km and a total area of about 10,000 km².

It is typified by extensive tidal mud flats, deeper tidal trenches and the islands that are contained within this, a region continually contested by land and sea. The landscape had been formed for a great part by storm tides.

The Wadden Sea is famous for the rich fauna, avifauna and flora. Today, a great part of the Wadden Sea is protected in cooperation of all three countries; see Wadden Sea National Parks for the protected areas within the German borders.

For the islands, see the applicable sections of List of islands.