Brazilian Portuguese (português brasileiro, português do Brasil) is the variety of the Portuguese language used in Brazil. It is written and spoken by virtually all of the 190 million inhabitants of Brazil and by a few million Brazilian emigrants, mainly in the United States, Paraguay, Japan, Portugal, and Argentina.

Some authors compare the differences between Brazilian Portuguese and European Portuguese to those found between American and British English. For others, these differences are even more extensive. The formal written language in Portugal and Brazil differs much less than the spoken varieties, and that difference has been further reduced by agreements on spelling reform in the 1940s, the early 1970s, and 1990.

In 1990 an agreement between Brazil and the other Lusophone countries was established to reduce the differences among their respective spelling conventions. The Portuguese Language Orthographic Agreement of 1990 (Portuguese: Acordo Ortográfico da Língua Portuguesa de 1990) is an international treaty whose purpose was meant to create a unified orthography for the Portuguese language, to be used by all the countries that have Portuguese as their official language. However, the agreement has not yet been ratified by all, due to a variety of specific problems.

In spite of the use of Brazilian Portuguese by people of various linguistic backgrounds, a number of other factors—especially its comparatively recent development and the cultural prestige and strong government support accorded to the written standard—have helped to maintain the unity of the language over the whole of Brazil and to ensure that all regional varieties remain mutually intelligible. Starting in the 1960s, the nationwide dominance of television networks based in the southeast (Rio de Janeiro and São Paulo) has made the accents of that region into a common spoken standard for the mass media, as well.