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 is formed where the Bergse Maas and Nieuwe Merwede rivers converge near Lage Zwaluwe, and near Numansdorp it splits 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 is characterized by deep channels, hard edges, shell beds, and broad plateaus, giving it a distinctly open, expansive character.

The Hollands Diep is a designated Ramsar Wetland (designated 29 August 2000, reference no. 1273), reflecting its ecological significance. On warm summer days the water sees a mix of recreational activity — canoes, motorboats, wakeboarders, and swimmers — giving it a lively, social atmosphere during the warmer months.

Paddlers should be aware that this is a large, dynamic waterway with significant commercial and recreational boat traffic. The Moerdijk bridges cross the eastern end, carrying road and rail traffic, and the Hellegatsplein road node sits at the western split point. The scale and open exposure of the water mean wind and current can be substantial factors, and paddlers should exercise caution accordingly. The water body has tidal influence, which is a relevant safety consideration for trip planning.

The surrounding area includes notable infrastructure such as the Haringvliet bridge and the Volkerak Sluices. The region sits within the broader Southwest Delta, which encompasses the Haringvliet, Volkerak, and Biesbosch — all accessible from or near the Hollands Diep. The town of Willemstad, located on the northern bank, has a boat ramp with free parking and serves as a practical access point for those arriving by water.

The water body's origins trace to the catastrophic St. Elizabeth's flood of 1421, which connected an earlier saltwater inlet to the Merwede and established the Hollands Diep as a major estuarine channel. This historical context gives the area considerable cultural and geographical significance within the Dutch delta landscape.