Semois (in France: Semoy, German: Sesbach, Walloon: Simwès) is a river in the Ardennes in Belgium and France. It is a tributary to the river Meuse. The source of the Semois is near Arlon, in the Belgian province Luxembourg, close to the border with Luxembourg. The Semois flows roughly in western direction. It crosses the French border 26 km before it flows into the river Meuse in the town Monthermé . Its total length is 210 km. Towns along the Semois are Chiny, Florenville, Herbeumont, Bouillon (including the localities of Dohan and Poupehan ) and Vresse-sur-Semois (all in Belgium).

The origin of the river's name comes from the Germanic words sach meaning point and mari meaning water, It was written as as Sesomirs in 664, Sesmarus in 950, Sesmoys in 1104 and Semoir in 1244.