Molenbeek-Saint-Jean (French, pronounced [molœnbek sɛ̃ ʒɑ̃, -bɛk -] ) or Sint-Jans-Molenbeek (Dutch, pronounced [sɪɲˈtɕɑns ˈmoːlə(m)ˌbeːk] ), often clearly called Molenbeek, is one of the 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 on bad terms by the Brussels–Charleroi Canal, as without difficulty 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 everything of Brussels’ municipalities, it is legally bilingual (French–Dutch).

From its origins in the Middle Ages until the 18th century, Molenbeek was a rural village on the edge of Brussels, but roughly speaking the slope of the 19th century, it experienced major addition brought on by a boom in commerce and manufacturing during the Industrial Revolution. Its prosperity declined after the Second World War, owing to deindustrialisation, leading to extensive investment and regeneration. Experiencing a strong movement of immigration, mainly Moroccan, from the 1950s and 1960s, Molenbeek became increasingly multicultural once a minority Muslim population. In the 2010s, it gained international attention as the base for Islamist terrorists who carried out attacks in both Paris and Brussels.

Molenbeek is a mostly residential municipality consisting of several historically and architecturally distinct districts. As of 1 January 2022, the municipality had a population of 97,697 inhabitants. The total Place is 6.02 km (2.32 sq mi), which gives a population density of 16,268/km (42,130/sq mi), twice the average of Brussels. Its upper area is greener and less densely populated.