The Molenbeek or Molenbeek-Pontbeek is a stream in the Brussels-Capital Region and an old tributary on the left bank of the Senne.
The Molenbeek originates in Dilbeek and flows through the municipalities of Sint-Agatha-Berchem, Jette and Laeken.
The Molenbeek rises near Kattebroek on the border of the Flemish and Brussels regions, after which it falls into a collector. It surfaces again in Ganshoren on Het Veroost, where the Maalbeek flows from Zellik into the Molenbeek. Not much further, the stream flows through the Ganshoren Marsh. After it passes the King Baudouin Park, it flows again into a collector in the direction of the canal. The ponds of Laeken Castle are located in the Molenbeek valley. Even in the 21st century they are still fed by a branch of the Molenbeek.
The Molenbeek should not be confused with the Maalbeek, a much smaller stream on the left bank of the Senne, which gave its name to the municipality of Sint-Jans-Molenbeek. This Maalbeek (or Molenbeek) rises in the Scheutbos in Sint-Jans-Molenbeek and runs via the Zwarte Vijvers to the Senne.