Braine-l'Alleud is a municipality of Wallonia located in the Belgian province of Walloon Brabant, about 20 kilometres south of Brussels. The Braine-l'Alleud municipality includes the former municipalities of Braine-l'Alleud proper, Ophain-Bois-Seigneur-Isaac, and Lillois-Witterzée. It also includes the hamlet of Sart-Moulin, the inverted name of which inspired Hergé’s Moulinsart castle. Bordering Flanders, the town is home to a minority of Dutch speakers.
The famous Lion of Waterloo, where the eponymous battle took place, is in the territory of Braine-l'Alleud.