The Cologne Stadtbahn is a light rail system in the German city of Cologne, including several surrounding cities of the Cologne Bonn Region. The term Stadtbahn denotes a system that encompasses elements of trams as well as an underground railway network (U-Bahn) and interurban rail, even including three lines that are licensed as heavy rail and used by freight trains as well as Stadtbahn vehicles. Two of these lines connect the Cologne Stadtbahn to the Bonn Stadtbahn. These lines are jointly operated by both cities' transport authorities, resulting in both systems and the lines connecting them sometimes collectively referred to as Stadtbahn Rhein-Sieg.

The Cologne Bonn Region is a metropolitan area in North Rhine-Westphalia (NRW), Germany, covering the cities of Cologne, Bonn and Leverkusen, as well as the districts of Rheinisch-Bergischer Kreis, Oberbergischer Kreis, Rhein-Erft-Kreis and Rhein-Sieg-Kreis. The region covers an area of 3,839 km² with 3.13 million inhabitants. The city centres of Cologne and Bonn are 24 kilometres apart as the crow flies. At the outer city limits, there are only 8 kilometres between Cologne-Libur and Bonn-Geislar.