The Allondon is a river in France (Auvergne-Rhône-Alpes region) and Switzerland (canton of Geneva). It is 17.9 km long, of which 6.1 km in Switzerland and 2.5 km on the French–Swiss border. Its source is located at 649 m elevation in the commune of Échenevex 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 148 km2, of which 137 km2 in France. It is home to the only recently known population of the rare insect Lachesilla rossica.