The North Sea is a marginal sea of the Atlantic Ocean located between Great Britain, Scandinavia, Germany, the Netherlands, and Belgium. An epeiric (or "shelf") sea on the European continental shelf, it connects to the ocean through the English Channel in the south and the Norwegian Sea in the north. It is more than 970 kilometres (600 mi) long and 580 kilometres (360 mi) wide, with an area of around 750,000 square kilometres (290,000 sq mi).

The North Sea has long been the site of important European shipping lanes as well as a major fishery. The sea is a popular destination for recreation and tourism in bordering countries and more recently has developed into a rich source of energy resources including fossil fuels, wind, and early efforts in wave power.

Historically, the North Sea has featured prominently in geopolitical and military affairs, particularly in Northern Europe but also globally through the power northern European actors projected worldwide during much of the Middle Ages and modern era. The North Sea was the center of the Vikings' rise and subsequently, the Hanseatic League, the Netherlands, and the British each sought to dominate the North Sea and through it to control access to the markets and resources of the world. As Germany's only outlet to the ocean, the North Sea continued to be strategically important through both World Wars.