The Hollands Diep (historically spelled Hollandsch Diep) is a large, open waterway in the southwestern Netherlands, functioning as an estuary of the Rhine and Meuse rivers. It was formed through a series of historic floods, most notably the St. Elizabeth's flood of 1421, which connected an earlier saltwater inlet to the Merwede and established the water body as a major estuarine channel. The Bergse Maas and Nieuwe Merwede rivers join near Lage Zwaluwe to form the Hollands Diep, which then splits near Numansdorp into the Haringvliet and the Volkerak. The Dordtsche Kil connects to it near Moerdijk, and through the Scheldt–Rhine Canal it links to the Scheldt river and Antwerp.

The water body is characterized by deep channels, hard edges, shell beds, and broad plateaus, giving it a distinctly open and dynamic character. It is a large, working waterway with significant infrastructure: the Moerdijk bridges cross the eastern end, carrying road and rail traffic, and the Hellegatsplein road junction sits on an artificial island at the western split. Paddlers should be aware that commercial vessel traffic is present on this water, and conditions can be demanding given the scale and exposure of the estuary.

On warm summer days, the Hollands Diep sees recreational use including canoeing, boating, wakeboarding, and swimming, suggesting it is accessible for paddlers during the summer months. The town of Willemstad, situated along the water, has a boat ramp with free parking, which serves as one practical access point for those arriving by water.

The Hollands Diep holds Ramsar Wetland designation, awarded in August 2000, reflecting its ecological significance. It is recognized as one of the more reliable waters in the region for predator fish species such as zander, pike, and perch, which points to a healthy aquatic ecosystem with varied bottom structure. Anglers fishing the water require a valid VISpas (Dutch fishing permit), and visitors should familiarize themselves with local regulations and any closed zones before launching. Paddlers planning a trip should account for the water's tidal influences, its size and exposure to wind, and the presence of commercial and recreational boat traffic throughout the navigable channels.