Molenbeek-Saint-Jean or Sint-Jans-Molenbeek, often simply called Molenbeek, is one of 19 municipalities of the Brussels-Capital Region, Belgium. Located in the western part of the region, it is bordered by the City of Brussels, from which it is separated by the Brussels–Charleroi Canal, as well as by the municipalities of Anderlecht, Berchem-Sainte-Agathe, Dilbeek, Jette and Koekelberg. The Molenbeek brook, from which it takes its name, flows through the municipality. In common with all of Brussels' municipalities, it is legally bilingual.
There are several historically and architecturally distinct districts within Molenbeek. As of 1 January 2020, the municipality had a population of 97,979 inhabitants. It is densely populated, at 16,314/km², twice the average of Brussels. Its upper area is greener and less densely populated.
In 2015, Molenbeek gained international attention as the base of Islamist terrorists, who carried out attacks in both Paris and Brussels. The municipality's then-mayor described it as "a breeding ground for violence".