The Sieg is a river in North Rhine-Westphalia and Rhineland-Palatinate, Germany. It is a tributary of the Rhine and 130 km in length.

The source is located in the Rothaargebirge mountains. From here the river runs southwestwards to the city of Siegen and the hills of Siegerland, both named after the river. Further west the Sieg valley forms the northern boundary of the Westerwald region. The river finally runs through the hills of the Bergisches Land and enters the industrial region surrounding the city of Bonn.

After passing the cities of Hennef and Siegburg it joins the Rhine just a few kilometres north of the centre of Bonn.