Gran Chaco or Chaco, region in south central South America, covering about 647,500 sq km (about 250,000 sq mi), and encompassing part of Argentina, Paraguay, and Bolivia. The Chaco is divided into three regions by its two main rivers, the Pilcomayo and the Bermejo. These regions are: the Chaco Boreal (Northern Chaco), the largest division located north of the Pilcomayo; the Chaco Central, located between the two rivers; and the Chaco Austral (Southern Chaco) located south of the Bermejo.

The Chaco is a low, flat, alluvial plain. It is generally hot and arid, but in the summer rainy season (December to April) it is subject to flooding by rivers that turn large parts of the area into swamp. The Pilcomayo and Bermejo traverse the Chaco in a southeastern direction from the Andean foothills toward the Paraguay and Paraná rivers. The hard clay that characterizes the soil aggravates the flooding and inhibits agriculture. Along the waterways in the east are tall reeds of palms; these give way inland to scrub forest and grassy savanna, or parkland; in the west are dense growths of spiny brush and desolate arid stretches. Wildlife is abundant and includes jaguar, ocelot, tapir, anteater, capybara, peccary, and agouti. Bird, reptile, and insect life is also diverse. The savanna is used for cattle ranching, which, with cotton growing in the south, has been developed with some success as an industry since the 1930s. The forested part of the Chaco in Paraguay and northern Argentina is noted for timber, especially the red quebracho logs from which come much of the world's supply of tannin.

Few roads and railroads traverse the area, which is sparsely populated. The principal towns are Mariscal Estigarribia, a Paraguayan army post, and Filadelfia, a Mennonite colony, both in the Paraguayan Chaco Boreal; various Paraguayan ports on the western bank of the Paraguay River, the largest of which is Villa Hayes; and the Argentine cities of Formosa and Resistencia. Dispute over boundaries led to armed conflict—the Chaco War (1932-1935)—between Bolivia and Paraguay.