The Union of South American Nations (Dutch: Unie van Zuid-Amerikaanse Naties ( pronunciation (help·info)) - UZAN, Portuguese: União de Nações Sul-Americanas - UNASUL, Spanish: Unión de Naciones Suramericanas - UNASUR) is an intergovernmental union integrating two existing customs unions: Mercosur and the Andean Community of Nations (CAN), as part of a continuing process of South American integration. It is modeled on the European Union.

The UNASUR Constitutive Treaty was signed on May 23, 2008, at the Third Summit of Heads of State, held in Brasília, Brazil. According to the Constitutive Treaty, the Union's headquarters will be located in Quito, Ecuador. The South American Parliament will be located in Cochabamba, Bolivia, while the headquarters of its bank, the Bank of the South are located in Caracas, Venezuela.

The combined population of the 12-member Union as at 1 July 2010 was estimated at 396,391,032.

On 4 May 2010, at an extraordinary heads of state summit held in Campana, 75 km (47 mi) north of Buenos Aires, former Argentine President Néstor Kirchner was unanimously elected the first Secretary-General of UNASUR for a two-year term. This new office was conceived as a first step towards the establishment of a permanent bureaucratic body for the supranational union, eventually superseding Mercosur and CAN political bodies. On 1 December 2010, Uruguay became the ninth nation to ratify the UNASUR treaty, thus giving the union full legality.