The Battle of the Ruhr was a strategic bombing campaign against the Ruhr Area in Nazi Germany carried out by RAF Bomber Command during the Second World War. The Ruhr was the main centre of German heavy industry with coke plants, steelworks, armaments factories and ten synthetic oil plants. The British attacked 26 targets identified in the Combined Bomber Offensive. Targets included the Krupp armament works (Essen), the Nordstern synthetic oil plant at Gelsenkirchen and the Rheinmetall–Borsig plant in Düsseldorf, which was evacuated during the battle. The battle included cities such as Cologne not in the Ruhr proper but which were in the larger Rhine-Ruhr region and considered part of the Ruhr industrial complex. Some targets were not sites of heavy industry but part of the production and movement of materiel.

The Ruhr, also referred to as the Ruhr area, sometimes Ruhr district, Ruhr region, or Ruhr valley, is a polycentric urban area in North Rhine-Westphalia, Germany. With a population density of 2,800/km2 and a population of over 5 million (2017), it is the largest urban area in Germany. It consists of several large cities bordered by the rivers Ruhr to the south, Rhine to the west, and Lippe to the north. In the southwest it borders the Bergisches Land. It is considered part of the larger Rhine-Ruhr metropolitan region of more than 10 million people, which is the third largest in Western Europe, behind only London and Paris.