Bochum (/ˈboʊxʊm/, BOHKH-uum; US: /-əm/ -⁠əm; German: [ˈboːxʊm] ^ⓘ; Westphalian: Baukem) is a city in North Rhine-Westphalia. With a population of 372,348 (April 2023), it is the sixth-largest city (after Cologne, Düsseldorf, Dortmund, Essen and Duisburg) in North Rhine-Westphalia, the most populous German federal state, and the 16th-largest city in Germany. On the Ruhr Heights (Ruhrhöhen) hill chain, between the rivers Ruhr to the south and Emscher to the north (tributaries of the Rhine), it is the second largest city of Westphalia after Dortmund, and the fourth largest city of the Ruhr after Dortmund, Essen and Duisburg. It lies at the centre of the Ruhr, Germany's largest urban area, in the Rhine-Ruhr Metropolitan Region, the second biggest metropolitan region by GDP in the European Union, and belongs to the region of Arnsberg. There are nine institutions of higher education in the city, most notably the Ruhr University Bochum (Ruhr-Universität Bochum), one of the ten largest universities in Germany, and the Bochum University of Applied Sciences (Hochschule Bochum).

The city lies on the low rolling hills of Bochum land ridge (Bochumer Landrücken), part of the Ruhrhöhen (highest elevations) between the Ruhr and Emscher rivers at the border of the southern and northern Ruhr coal region. The highest point of the city is at Kemnader Straße (Kemnader Street) in Stiepel at 196 metres (643 ft) above sea level; the lowest point is 43 metres (141 ft) at the Blumenkamp in Hordel.

The terrain of Bochum is characterised by rolling hills that rarely have more than three per cent graduation. Steeper graduation can be found at the Harpener Hellweg near the Berghofer Holz nature reserve (3.4%), at Westenfelder Straße in the borough of Wattenscheid (3.47%), or at Kemnader Straße, which begins at the banks of the Ruhr in Stiepel (71 m, 233 ft), and rises to its highest point in the centre of Stiepel (196 m, 643 ft, a 5.1% increase).

The city extends north to south 13.0 km (8.1 mi) and 17.1 km (10.6 mi) east to west. The perimeter of the city limits is 67.2 km (41.8 mi).

It is surrounded by the cities of (in clockwise direction) Herne, Castrop-Rauxel, Dortmund, Witten, Hattingen, Essen and Gelsenkirchen.

There is sedimentary rock of carbon and chalk. The geological strata can be visited in the former quarry of Klosterbusch in Querenburg and at the Geological Gardens in Wiemelhausen.

The urban area is divided into the river Ruhr catchment in the south and the Emscher catchment in the north. The Ruhr's tributaries are the Oelbach (where as well a waste water treatment plant is established), Gerther Mühlenbach, Harpener Bach, Langendreer Bach, Lottenbach, Hörsterholzer Bach and the Knöselbach. The Ruhr in combination with upstream reservoirs is also used for drinking water abstraction. The Emscher's tributaries are Hüller Bach with Dorneburger Mühlenbach, Hofsteder Bach, Marbach, Ahbach, Kabeisemannsbach and Goldhammer Bach. The industrial developments in the region since the 19th century were leading to a kind of division of labour between the two river catchments, pumping drinking water from the Ruhr into the municipal supply system and discharging waste water mainly into the Emscher system. Today approximately 10% of the waste water in the Emscher catchment is discharged via the Hüller Bach. and treated in the centralized waste water treatment plant of the Emschergenossenschaft in Bottrop. The ecological restoration of the Emscher tributaries initiated by the Emschergenossenschaft started with the Internationale Bauausstellung Emscher Park in 1989.

The south of the city has woods, the best known of which are the Weitmarer Holz [de]. These are generally mixed forests of oak and beech. The occurrence of holly gives evidence of Bochum's temperate climate. 844 species of plants can be found within the city limits