The Lek is a river in the western Netherlands of some 60 km in length. It is the name of a distributary branch of river Rhine, which, shortly after entering the Netherlands, bifurcates into river Waal to the southwest and river Nederrijn (Nether Rhine) to the northwest.

At the town of Wijk bij Duurstede, the Kromme Rijn stream forks off to the northwest towards the city of Utrecht with the main westbound waterway becoming river Lek. A short distance past Wijk, the river intersects with the Amsterdam-Rijnkanaal (Amsterdam-Rhine Canal), which continues south towards the Waal. A branch of this canal, the Lekkanaal ("Lek Canal") is connected to the river at the town of Nieuwegein.

Other major towns on its banks are Culemborg, Vianen, Schoonhoven, Nieuw-Lekkerland and Lekkerkerk. The bed of the river lies slightly higher than the surrounding lands and dikes are therefore essential to containing the Lek. At the village of Kinderdijk the Lek meets the Noord river and the combined stream is thereafter known as the Nieuwe Maas as it flows down towards the North Sea.