The Paraguay River (Río Paraguay in Spanish, Rio Paraguai in Portuguese, Ysyry Paraguái in Guarani) is a major river in south-central South America, running through Brazil, Bolivia, Paraguay and Argentina. It flows about from its headwaters in the Brazilian state of Mato Grosso to its confluence with the Paraná River north of Corrientes and Resistencia.
The Paraguay's source is south of Diamantino in the Mato Grosso state of Brazil. It follows a generally southwesterly course, passing through the Brazilian city of Cáceres. It then turns in a generally southward direction, flowing through the Pantanal wetlands, the city of Corumbá, then running close to the Brazil-Bolivia border for a short distance in the Brazilian states of Mato Grosso and Mato Grosso do Sul.
From the city of Puerto Bahia Negra, Paraguay, the river forms the border between Paraguay and Brazil, flowing almost due south before the confluence with the Apa River.
The Paraguay makes a long, gentle curve to the south-southeast before resuming a more south-southwesterly course, dividing the country of Paraguay into two distinct halves: the Gran Chaco region to the west, a largely uninhabited semi-arid region; and the eastern forested departments of the country, accounting for some 98% of the country's inhabitants. As such the river is considered perhaps the key geographical feature of the country with which it shares its name.
Some after flowing through the middle of Paraguay, at the confluence with the Pilcomayo River and passing the Paraguayan capital city, Asunción, the river forms the border with Argentina, flowing generally south-southwesterly for another before it reaches its end, joining with the Paraná River.
The Paraguay River is the second major river of the Rio de la Plata Basin, after the Paraná River. The Paraguay's drainage basin, about , covers a vast area that includes major portions of Argentina, southern Brazil, parts of Bolivia, and most of the country of Paraguay.