Guaraní (Avañe'ẽ, 'language of the people') belongs to the Tupí–Guaraní family, one of the branches of the large Tupian linguistic stock of South America. It is part of a continuum of Guaraní peoples who spread across the Paraná–Paraguay basin long before European arrival. Modern Paraguayan Guaraní descends from the varieties spoken in the region and is distinct from other still-living ethnic Guaraní varieties such as Mbyá, Avá and Paĩ Tavyterã.

Guaraní is exceptional in being spoken by the majority of Paraguay's population, both Indigenous and non-Indigenous, making it one of the few Amerindian languages spoken by a largely mestizo national society. It is also spoken in parts of Argentina (Corrientes), Bolivia and Brazil, totaling several million speakers. Its historical territory spans eastern Paraguay and neighboring river basins, with a culture tied to the forest, the rivers, and a rich oral tradition.

During the colonial period, Guaraní was intensively documented by the Jesuits, especially in the famous reductions or missions that flourished in the seventeenth and eighteenth centuries. Father Antonio Ruiz de Montoya produced fundamental works such as the Tesoro de la lengua guaraní (1639) and a grammar and vocabulary that early on fixed its writing. In these missions Guaraní was used in writing, liturgy and even local printing presses, giving it a prestige and continuity unusual for an Indigenous American language.

Guaraní has been an official language of Paraguay alongside Spanish since the 1992 Constitution, and is also one of the official languages of Mercosur. It enjoys exceptional vitality, with broad intergenerational transmission and everyday use that gives rise to jopara, a fluid mixture of Guaraní and Spanish. There is an Academy of the Guaraní Language, compulsory bilingual education, and literary and media output, although tensions persist between popular Guaraní and the academic standard.