The Yser (US: /iːˈzɛər/ ee-ZAIR,[1] French: [izɛʁ]; Dutch: IJzer [ˈɛizər] ⓘ) is a river that rises in French Flanders (the north of France), enters the Belgian province of West Flanders and flows through the Ganzepoot and into the North Sea at the town of Nieuwpoort.

During the Battle of the Yser in the First World War, by opening the sluices, part of the polder west of the Yser was flooded with seawater between Nieuwpoort and Diksmuide to provide an obstacle to the advancing German Army and keep westernmost Belgium safe from German occupation.

The Yser rises west of Cassel in northern France and has two official sources, at Buisscheure and Lederzele.

In Belgium, the Poperinge canal, Kemmelbeek, Lovaart, Ieperlee and Handzamevaart join the river.

At the end of the 16th century this came into Spanish hands and a few settlements/forts were erected along the Yser against plundering by reformers from Ostend.

Like some other names in Flanders, the name IJzer is of Celtic origin, as the Celtic Menapii lived in the area before the Roman times (see also the Isar in Germany, the Jizera in Bohemia and the Isère in France) before assimilation by the Franks.