Guaraní, and more specifically its primary variety known as Paraguayan Guaraní, (IPA: /ɡwɑrəˈniː/; local name avañe'ẽ [aʋaɲẽˈʔẽ]), is an indigenous language of South America that belongs to the Tupí-Guaraní subfamily of the Tupian languages. It is one of the official languages of Paraguay (along with Spanish), where it is spoken by 94% of the population. It is also spoken by indigenous communities in neighbouring countries, including parts of northern Argentina, eastern Bolivia and southwestern Brazil. It is also treated as a second official language of the Argentine provinces of Corrientes and Misiones.

It includes a number of "dialects" now generally classified as distinct languages: Chiripá, Eastern Bolivian or Western Argentine Guaraní, Mbyá Guaraní, Paraguayan Guaraní, etc. Of these, Paraguayan Guaraní is by far the most important variety and it is often referred to simply as Guaraní.

Guaraní is the only indigenous language of the Americas whose overwhelming majority of speakers are non-indigenous people. This is an anomaly in the Americas where language shift towards more prestigious official languages (in this case, Spanish) has otherwise been a nearly universal cultural and identity marker of mestizos (people of mixed Spanish and Amerindian ancestry), and also of culturally assimilated, upwardly-mobile Amerindian people.

Jesuit priest Antonio Ruiz de Montoya, who wrote a book called Tesoro de la lengua guaraní ("The Treasure of the Guaraní Language"), described Guaraní as a language "so copious and elegant that it can compete with the most famous [of languages]."