The Rouillon is a small river in southern Île-de-France (France), left tributary of the Yvette, which is a tributary of the Orge. Its source is in Villejust, north of Longjumeau, in the Essonne department.The Rouillon crosses the following départements and towns:Essonne: Villejust, Saulx-les-Chartreux, Ballainvilliers, Longjumeau.