The Hermance () is a river in France and Switzerland, that discharges into Lake Geneva. It rises near Loisin in Haute-Savoie, France, at elevation. In its lower half, downstream from Veigy, it marks the Swiss-French border. It flows into Lake Geneva between the villages Hermance (Switzerland) and Chens-sur-Léman (France), at elevation. Its total length is ( according to the Sandre database), of which 6.5 kilometres on the border. Its catchment area is , of which 37 km<sup>2</sup> in France.
Flood control and other river maintenance issues are regulated under the 1959 Hermance River Agreement.