The Cologne Stadtbahn is a light rail system in the German city of Cologne, including several surrounding cities of the Cologne Bonn Region (Bergisch Gladbach, Bonn, Bornheim, Brühl, Frechen, Hürth, Leverkusen-Schlebusch, Wesseling). 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 (16 and 18) 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 Stadtbahn is operated by the Kölner Verkehrsbetriebe (KVB) and the Bonn Stadtbahn is operated by the Stadtwerke Bonn (SWB – City of Bonn Utilities Division). The KVB and SWB are members of the Verkehrsverbund Rhein-Sieg (VRS – Rhein-Sieg Transit Authority), formed in 1987 to consolidate the transit authorities in the metropolitan Cologne area and operate a joint fare structure. There are 236 stations along 199 km (124 mi) of tracks, of which 42 are underground in 28 km (17 mi) of tunnels.

The Cologne Stadtbahn traces its history to the first horsecar lines that started operating in 1877. Within a few years, several companies had built an extensive network. Because none of these companies showed interest in electrifying their lines, the city of Cologne bought them on 1 January 1900, and by 1907 all horsecar lines had been electrified or replaced by other services. Additional lines were built until 1912, including Vorortbahnen to surrounding villages outside the city limits. Outside the city center, these lines had segregated right-of-way and were more similar to "real" railroads than to trams.

The dense tram network in the city center that had been present before World War II was only partially rebuilt afterwards, and the Hauptbahnhof (Central Station) was not linked to important places like Neumarkt and Heumarkt for the time being. This was partly due to the plans to make Cologne a car friendly city which meant that old tramways (like the north–south one) were replaced by bus service; and also due to the planned widening of vehicles which wouldn't have fit through some of the narrow streets previously served by trams. Another question was the connection between the two banks of the Rhine which had been provided by the Hohenzollern-, Mülheim- and Deutzer Brücke, that were all destroyed during the war. A decision was taken to dedicate the Hohenzollern Bridge solely to interregional services, while the new Severins Bridge was only finished in 1959, allowing a restructuring of tram lines.

Already in 1956, the transportation plan of the city council intended a newly built tunnel through the city center in the north–south direction. An overground routing along the Nord-Süd-Fahrt (North-South-Road) would not have served many central areas and was rejected. The tunnel was not planned as a U-Bahn as in many other cities, but rather as an underground tram. This meant a design similar to overground trams with small turning radii, short distances between stops and low-floor platforms. At the end of the tunnels the trains were supposed to run as trams on the streets again, but in many places the trams were already separated from car traffic anyways. For this type of system – tunnels in highly congested areas, elsewhere mostly on or next to streets – the term Stadtbahn was later created. Similar concepts to separate rail and car traffic were later used in many cities in North Rhine-Westphalia and adjacent foreign cities (Brussels, Antwerp).

The term Stadtbahn is not regularly used, as the system is mostly referred to as U-Bahn and sometimes Straßenbahn (tram) by officials and the KVB, and most underground stations are also marked by a big white U on blue background which refers to U-Bahn. Most people in Cologne speak about the U-Bahn or Straßenbahn, or just about the train or the KVB.

When construction of the first tunnel started in 1963, there was no promise of the state NRW or the federal government to finance the project, and in the worst case the city would have had to pay completely on their own. With this background, the building of the Innenstadttunnel (city center tunnel) was begun. The tunnel opened in parts between 1968 and 1970 and was designed to be continued later, when more money and higher demand could make it necessary. This was the reason why no traditional U-Bahn (metro) was built in Cologne, which is as of 2023 the largest city in Europe not to have one. Nearly all tunnels completed afterwards aren't new routes, but rather overground sections of existing lines moved to the underground.

Beneficial to the expansion of the Stadtbahn were the Vorortbahnen that mostly ran on the right side of the Rhine and usually had their own right-of-ways. Another advantage was the previous use of 2.5 m (8 ft 2 in) wide trains instead of the usual 2.3 m (7 ft 7 in) other cities used for their trams, which made the adjustment for the new 2.65 m (8 ft 8 in) wide Stadtbahnwagen B easier.^{[citation needed]}