The Yser is a Belgian border river that forms part of the border between Belgium and France. It is a tributary of the North Sea, and is approximately 80 km long. The Yser originates in the French region of Nord-Pas-de-Calais and flows through the Belgian province of West Flanders before emptying into the North Sea near Nieuwpoort. The Yser is an important source of water for the region, and is also used for recreational activities such as fishing and boating.