The Sonian Forest is a 4,421-hectare (10,920-acre) forest that lies across the south-eastern part of Brussels, Belgium. The forest lies in the Flemish municipalities of Sint-Genesius-Rode, Hoeilaart, Overijse and Tervuren, in Uccle, Watermael-Boitsfort, Auderghem and Woluwe-Saint-Pierre in the Brussels-Capital Region and in the Walloon towns of La Hulpe and Waterloo.
Thus it stretches out over the three Belgian Regions. It is maintained by Flanders (56%), the Brussels-Capital Region (38%) and Wallonia (6%). There are some contiguous tracts of privately-held forest and the Kapucijnenbos, the "Capuchin Wood", which belongs to the Royal Trust. The forest is part of the scattered remains of the ancient Silva Carbonaria or Charcoal Forest. The first mention of the Sonian Forest (Soniaca Silva) dates from the early Middle Ages. Then the forest south of Brussels was crossed by the river Zenne/Senne and extended as far as Hainaut, covering most of the high ground between the Zenne and the Dijle. The ninth-century vita of Saint Foillan mentions "the forest, next to the abbey of Saint Gertrude, called the Sonesian" In the sixteenth century it was still seven leagues in circumference. At the start of the 19th century the area of the wood was still about 100 square kilometres, but due to wood cutting its area diminished to its current area of 44.21 km².