Hollands Diep is a wide river in the Netherlands and an estuary of the Rhine and Meuse river. Through the Scheldt-Rhine Canal it connects to the Scheldt river.

The Bergse Maas river and the Nieuwe Merwede river join near Lage Zwaluwe to form the Hollands Diep. The Dordtsche Kil connects to it at near Moerdijk. Near Numansdorp it splits into the Haringvliet and the Volkerak . At this point there is the Hellegatsplein in the water, a road traffic node, connecting to the Hoeksche Waard island on the north by the Haringvliet bridge, to the Goeree-Overflakkee on the west by the Hellegatsdam, and to the mainland on the southeast by the Volkerak Sluices.

On the east side of the Hollands Diep the Moerdijk bridges connect the Dordrecht island on the north to the mainland on the south: a road bridge, a railroad bridge; a second railroad bridge has recently been constructed for the HSL.