The Oude Rijn (Old Rhine) is a branch of the Rhine delta in the Dutch provinces of Utrecht and South Holland. Its present-day length is 52 kilometres. In Roman times, it was the main Rhine branch, forming the northern border of the Empire. The river silted up in the course of the Middle Ages and had lost all of its importance by the seventeenth century.

The city of Utrecht was founded at a ford near the fork of river Kromme Rijn into the Vecht to the north, and the Oude Rijn to the west. Of the original fork, little remains today and both Vecht and Rijn start from the city moat. For the first few kilometres of its course, the river is channelised and known as Leidse Rijn ("Leyden Rhine"). It flows past the towns of De Meern , Harmelen, Woerden, Nieuwerbrug , Bodegraven, Zwammerdam, Alphen aan den Rijn, Koudekerk aan den Rijn , Leiderdorp and Leiden towards Katwijk. Here, the Oude Rijn originally flowed into the North Sea, but the stream has been closed off by a sluice complex regulating its water level.

Many of the cities and villages along the Oude Rijn are of Roman origin, stemming from border-guarding castella and castra.