The Lesse is an Ardennes river of Belgium, tributary on the right bank of the Meuse.
The length of its river is 89 km. Its source is in Ochamps, in the commune of Libin, west of Libramont, in the province of Luxembourg. The Lesse flows globally in a north-westerly direction.
Penetrating into the limestone region of Calestienne near Han-sur-Lesse (commune of Rochefort), the river rushes underground to the abyss of Belvaux and makes its way through the caves of Han. The route of the Lesse from its source to the abyss of Belvaux corresponds to the Haute-Lesse.
Several of the caves along the river are important archaeological sites - including Trou de Chaleux (at the Circus of Chaleux), and Trou du Nutons and Frontal Hole at Furfooz. The river flows into the Meuse in the village of Anseremme, in the commune of Dinant. The most famous villages along the Lesse are Daverdisse, Han-sur-Lesse, Houyet and Anseremme.