Casteau (Walloon: Castea) originated as a village in the Hainaut province of Wallonia, in the French speaking south of Belgium. Casteau has become a district of the municipality of Soignies, centred around Soignies (town), and together with the former villages, now districts: Chaussée-Notre-Dame-Louvignies, Horrues, Naast, Neufvilles, and Thieusies.
The village is located between the towns of Mons and Soignies on the Mons-Brussels road. The river Obrecheuil flows through the village. In the past, there were some watermills along the river.