The Sonian Forest or Sonian Wood is a 4,421-hectare forest at the southeast edge of Brussels, Belgium.
The forest lies in the Flemish municipalities of Sint-Genesius-Rode, Hoeilaart, Overijse, and Tervuren, in the Brussels-Capital Region municipalities of Uccle, Watermael-Boitsfort, Auderghem, and Woluwe-Saint-Pierre, and in the Walloon towns of La Hulpe and Waterloo. Thus, it stretches out over the three Belgian Regions.
It is maintained by Flanders, the Brussels-Capital Region, and Wallonia. There are some contiguous tracts of privately held forest and the Kapucijnenbos, the "Capuchin Wood", which belongs to the Royal Trust.