North Sea, a body of water between Great Britain and continental Europe. It merges on the north with the Norwegian Sea (an arm of the Atlantic Ocean) and is bounded on the east by Norway and Denmark; on the south by Germany, the Netherlands, Belgium, and the northern tip of France; and on the west by Great Britain. The sea is connected to the English Channel (and to the Atlantic Ocean) by the Strait of Dover, in the southwest. On the east it is joined to the Baltic Sea by a series of straits, the largest being the Skagerrak and the Kattegat. The Kiel Canal, passing through the German part of the Jutland peninsula, also joins the two seas.

The North Sea is some 725 miles (1,170 km) long, north-south, and from 125 to 450 miles (200 to 725 km) wide. The sea is relatively shallow, averaging less than 400 feet (120 m) in depth except off the Norwegian coast. It has a surface area of about 220,000 square miles (570,000 km2). Major rivers flowing into the sea are the Elbe, Rhine, Weser, and Thames.

The North Sea is a transportation and trade route and a fishing ground. On this sea travel ships from some of the world's largest ports—London, Antwerp, Hamburg, Rotterdam, and Bremen. Navigation is often difficult because of coastal sand banks, frequent fogs, and violent storms. However, the sea does not freeze over. Productive fishing areas are off the European coast and in the Dogger Bank—a shallow area in the south-central part of the sea. These areas supply most of northern Europe and the United Kingdom with herring, mackerel, haddock, and cod. Deposits of petroleum and natural gas underlie the sea's floor, and Great Britain and Norway have become major producers of these resources.

This sea has been the site of historic naval battles. One of the earliest resulted in the defeat of English forces by the Dutch admiral de Ruyter in the 17th century. During World War I German submarines menaced Allied shipping here, and several battles between British and German forces were fought—including the Battle of Jutland in 1916, the major naval engagement of the war. The North Sea was heavily mined late in the war to hamper German submarines. British and German warships ranged the sea again during World War II, but confined much of their activity to laying mines and patrolling the waters.