Gelsenkirchen is the 25th most populous city of Germany and the 11th most populous in the state of North Rhine-Westphalia with 262,528 (2016) inhabitants. On the Emscher River, it lies at the centre of the Ruhr, the largest urban area of Germany, of which it is the fifth largest city after Dortmund, Essen, Duisburg and Bochum. The Ruhr is located in the Rhine-Ruhr Metropolitan Region, one of Europe's largest urban areas. Gelsenkirchen is the fifth largest city of Westphalia after Dortmund, Bochum, Bielefeld and Münster, and it is one of the southernmost cities in the Low German dialect area. The city is home to the football club Schalke 04, which is named after Gelsenkirchen-Schalke. The club's current stadium Veltins-Arena, however, is located in Gelsenkirchen-Erle.

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.