Buenos Aires (Spanish pronunciation: [ˈbwenoˈsaiɾes]) is the capital and largest city of Argentina, and the second-largest metropolitan area in South America, after São Paulo. It is located on the western shore of the estuary of the Río de la Plata, on the southeastern coast of the South American continent. Greater Buenos Aires conurbation, which also includes several Buenos Aires Province districts, constitutes the third-largest conurbation in Latin America, with a population of around 13 million.

The city of Buenos Aires is not a part of the Buenos Aires Province, nor is it its capital; rather, it is an autonomous district. In 1880, after decades of political infighting, Buenos Aires was federalised and removed from Buenos Aires Province. The city limits were enlarged to include the towns of Belgrano and Flores (both are currently neighbourhoods of the city). The 1994 constitutional amendment granted the city autonomy, hence its formal name: Ciudad Autónoma de Buenos Aires (English: Autonomous City of Buenos Aires). Its citizens first elected a Chief of Government (i.e. Mayor) in 1996; before, the Mayor was directly appointed by the President of the Republic.

Buenos Aires is considered an Alpha World City as listed by the Loughborough University group's (GaWC) 2008 inventory. People from Buenos Aires are referred to as porteños (people of the port).