On 1 January 2006 the municipality had a total population of 24,056. Its total area is 12.93 km² which gives it a population density of 1,860 inhabitants per km². It is the lowest population density of the 19 communities in the Brussels-Capital Region.
Watermael-Boitsfort is in the south of the capital and borders the communes of Uccle (Ukkel), Auderghem (Oudergem) and Ixelles (Elsene) as well as the province of Flemish Brabant. The Sonian Forest (Zoniënwoud or Forêt de Soignes) covers almost 60% of the municipality.