The Wadden Sea (Dutch: Waddenzee [ˈʋɑdə(n)zeː] ⓘ; German: Wattenmeer [ˈvatn̩ˌmeːɐ̯] ⓘ; 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 low-lying Frisian Islands, forming a shallow body of water with tidal flats and wetlands.

[8] The landscape has been formed for a great part by storm tides in the 10th to 14th centuries, overflowing and carrying away former peat land behind the coastal dunes.

The impact of waves and currents carrying away sediments is slowly changing both land masses and coastlines.

For example, the islands of Vlieland and Ameland have moved eastwards through the centuries, having lost land on one side and added it on the other.

Hundreds of thousands of waders, ducks, and geese use the area as a migration stopover or wintering site.

It is also a rich habitat for gulls and terns,[10] as well as a few species of herons, Eurasian spoonbills and birds-of-prey, including a small and increasing breeding population of white-tailed eagles.

The world's only remaining natural population of houting survives in the Danish part of the Wadden Sea and it has been used as a basis for reintroductions further south, but considerable taxonomic confusion remains over its status (whether it is the same as the houting that once lived further south in the Wadden Sea).

[24][25] They are generally considered long-extinct in the region, but in the Netherlands, a possible right whale was observed close to beaches on Texel in the West Frisian Islands and off Steenbanken, Schouwen-Duiveland in July 2005.

Mudflat hiking, i.e., walking on the sandy flats at low tide, has become popular in the Wadden Sea.

The German part of the Wadden Sea was the setting for the 1903 Erskine Childers novel The Riddle of the Sands and Else Ury's 1915 novel Nesthäkchen in the Children's Sanitorium.

The latter term is generally understood to include all coastal regions around the Wadden Sea that participate in the trilateral cooperation between Denmark, Germany and the Netherlands.