The Sieg is a river in North Rhine-Westphalia and Rhineland-Palatinate, Germany named after the Sigambrer. It is a right tributary of the Rhine and 153 kilometres 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 boundary of the Bergisches Land (northern) and Westerwald (southern). The river finally runs through a protected area east of the city of Bonn.
After passing the cities of Hennef and Siegburg it joins the Rhine near Niederkassel/Mondorf just a few kilometres north of the centre of Bonn.