The Allondon () is a river in France (Auvergne-Rhône-Alpes region) and Switzerland (canton of Geneva). It is long, of which 6.1 km in Switzerland and 2.5 km on the FrenchâÂÂSwiss border. Its source is located at elevation in the commune of ÃÂchenevex in the Ain department in eastern France. It flows into the Rhône at the village of Russin in the canton of Geneva in Switzerland. Its catchment area is , of which 137 km<sup>2</sup> in France. It is home to the only recently known population of the rare insect Lachesilla rossica.