Guarani are a group of culturally related indigenous peoples of South America. They are distinguished from the related Tupí by their use of the Guaraní language. The traditional range of the Guaraní people is in what is now Paraguay between the Uruguay River and lower Paraguay River, the Misiones Province of Argentina, southern Brazil once as far as north as Rio de Janeiro, and parts of Uruguay and Bolivia. Although their demographic dominance of the region has been reduced by European colonisation and the commensurate rise of mestizos, there are contemporary Guaraní populations in these areas. Most notably, the Guarani language, still widely spoken across traditional Guaraní homelands, is one of the two official languages in Paraguay, the other one being Spanish. The language was once looked down upon by the upper and middle classes, but it is now often regarded with pride and serves as a symbol of national distinctiveness. The Paraguayan population learns Guaraní both informally from social interaction and formally in public schools. In modern Spanish Guaraní is also applied to refer to any Paraguayan national in the same way that the French are sometimes called Gauls.

Tupí–Guaraní is the name of the most widely distributed subfamily of the Tupian languages of South America. It includes fifty languages, including the best-known languages of the family, Guaraní and Old Tupí.

The Tupí people were one of the most important indigenous peoples in Brazil. Scholars believe they first settled in the Amazon rainforest, but 2900 years ago they started to spread southward and gradually occupied the Atlantic coast.