Johannesburg is the most populous city in South Africa with 4,803,262 people, and is classified as a megacity; it is one of the 100 largest urban areas in the world. It is the provincial capital and largest city of Gauteng, which is the wealthiest province in South Africa. Johannesburg is the seat of the Constitutional Court, the highest court in South Africa. Most of the major South African companies and banks have their head offices in Johannesburg. The city is located within the mineral-rich Witwatersrand hills, the epicentre of the international-scale mineral, gold and (specifically) diamond trade.

A megacity is a very large city, typically with a population of more than 10 million people. The United Nations Department of Economic and Social Affairs in its 2018 "World Urbanization Prospects" report defines megacities as urban agglomerations with over 10 million inhabitants. A University of Bonn report holds that they are "usually defined as metropolitan areas with a total population of 10 million or more people". Elsewhere in other sources, from five to eight million is considered the minimum threshold, alongside a population density of at least 2,000 per square kilometre. The terms conurbation, metropolis, and metroplex are also applied to the latter.