Gran Chaco, or Chaco, a low-lying plains region in south-central South America. It lies mainly in Paraguay and Argentina and extends into Bolivia and Brazil, covering 250,000 square miles (647,000 km2). The Chaco divides into the Chaco Boreal, in the north; the Chaco Central; and the Chaco Austral, in the south. Scrubby woodlands occupy most of the region. Swamps prevail along many rivers, especially in the east, where flooding is often severe. Winters are mild and summers very hot.

The Chaco affords little opportunity for economic development and is sparsely settled. Cattle raising, the cutting of quebracho trees (for tannin), and cotton-growing (in Argentina) are the main activities. Disputes between Bolivia and Paraguay over the Chaco Boreal led to the Chaco War of 1932–35. A treaty signed in 1938 awarded most of this area to Paraguay.