The Sonian Forest is a large forest in the south-eastern part of Brussels, Belgium. The forest is in the Flemish parts of Sint-Genesius-Rode, Hoeilaart, Overijse and Tervuren, Uccle , Watermael-Boitsfort, Auderghem and Woluwe-Saint-Pierre in the Brussels-Capitale Region and in the Walloon towns of La Hulpe and Waterloo. Thus it stretches out over the three Belgian Regions. It used to be connected with Bois de la Cambre and Hallerbos (Blue Forest in Belgium) but they split up in 1900.
It is maintained by Flanders (56%), the Brussels-Capitale Region (38%) and Wallonia (6%). There are some parts of the forest privately held forest and the Kapucijnenbos, the "Capuchin Wood", which belongs to the Royal Trust.