Mercosur (Spanish: Mercado Común del Sur, Portuguese: Mercado Comum do Sul, English: Southern Common Market) is a Regional Trade Agreement (RTA) among Argentina, Brazil, Paraguay and Uruguay founded in 1991 by the Treaty of Asunción, which was later amended and updated by the 1994 Treaty of Ouro Preto. Its purpose is to promote free trade and the fluid movement of goods, people, and currency.

Mercosur origins trace back to 1985 when Presidents Raúl Alfonsín of Argentina and José Sarney of Brazil signed the Argentina-Brazil Integration and Economics Cooperation Program or PICE (Spanish: Programa de Integración y Cooperación Económica Argentina-Brasil, Portuguese: Programa de Integração e Cooperação Econômica Argentina-Brasil). The program also proposed the Gaucho as a currency for regional trade.

Bolivia, Chile, Colombia, Ecuador and Peru currently have associate member status. Venezuela signed a membership agreement on 17 June 2006, but before becoming a full member its entry has to be ratified by the Paraguayan and the Brazilian parliaments.