Wilhelmshaven is a very young town, since it was founded in 1869 by the Prussian king Wilhelm I. There was a castle at the place as early as 1383, the Sibetsburg; this castle was owned by pirates and destroyed in 1433 by the Hanseatic League. Then the site of today's Wilhelmshaven was uninhabited, until four centuries later the kingdom of Prussia planned to establish an own fleet and a harbour at the North Sea. In 1853 Prussia and the grand duchy of Oldenburg entered into a contract: 313 hectare of Oldenburgian territory at the Jadebusen should be ceded to Prussia. Wilhelm I (later to become the German emperor of the same name) inaugurated the port in 1869 and gave it his name.
Wilhelmshaven grew to become one of the most important German ports. The duchy of Oldenburg benefited as well from the port; adjacent to Wilhelmshaven the town of Rüstringen was founded on the Oldenburgian territory. Wilhelmshaven and Rüstringen were merged in 1937.
In World War II the city, which was occupied by the German navy, was bombed by Allied air attacks; two thirds of the town's buildings were destroyed. After the war the harbour was used not only for military purposes, but for economy and tourism as well. Today Wilhelmshaven is the second largest German port (after Hamburg).
In Wilhelmshaven there is the German Navy Museum, an aquarium with native animals from the North Sea, the information centre of the Lower Saxony Wadden Sea National Park. The town's landmark is the Kaiser-Wilhelm-Brücke ("Emperor Wilhelm Bridge"), which crosses an inlet of the Jadebusen. It was built in 1908; with a length of 159 m it was once the greatest swing bridge of Europe.