Guaycuru or Guaykuru is a generic term for several ethnic groups indigenous to the Gran Chaco region of South America, speaking related Guaicuruan languages.

In the 16th century, the time of first contact with Spanish explorers and colonists, the Guaycuru people lived in the present-day countries of Argentina (north of Santa Fe Province), Paraguay, Bolivia, and Brazil (south of Corumbá).

When first encountered in the 16th century, the Guaycuru lived in the Gran Chaco, an inhospitable region for agriculture and settlement in the eyes of the Spanish colonists.

The governor of Paraguay, Alvar Núñez Cabeza de Vaca, said in the 1540s of the Guaycuru :"These Indians are great warriors and valiant men, who live on venison, butter, honey, fish, and wild boar...They go daily to the chase for it is their only occupation.

They and other Guaycuruans acquired horses and cattle by raiding Spanish haciendas and Guaraní settlements and Jesuit missions east of the Paraguay and Parana rivers.

Between raids they traded skins, wax, honey, salt, and Guaraní slaves to the Spanish en exchange for knives, hatchets, and other products.

The mobility afforded by the horse facilitated Guaycuruan control over other peoples in the Chaco and made raiding the Spaniards and their Indian allies a profitable enterprise.

The Payagua plied the river in canoes, fished and gathered edible plants, and raided their agricultural neighbors, the Guaraní, to the east.

The bands only united on ceremonial occasions, especially during the harvest period for wild honey and algarroba (Prosopis) pods which were used to produce a fermented alcoholic beverage.

He dispatched a large expedition of Spaniard and Guaraní soldiers from Asunción and attacked an encampment of Mbayas, also called Eyiguayegis.

In the aftermath of the battle, however, the Guaycuruans retained their control of the Chaco and gradually acquired horses, a taste for Spanish beef, and iron weapons and tools.

[15] The Mbayas were given land by Brazil for their assistance in the Paraguayan War (1864-1870), but survive only as the Kadiweu, numbering 1,400 in 2014..[16] The still-nomadic Tobas and Mocovis in the Argentine Chaco continued to resist the advancing frontier until 1884, when they were decisively defeated by the army; while a number of them agreed to thereafter live in reductions, thousands of Tobas retreated to isolated regions of Argentina, Paraguay and Bolivia and retained some level of autonomy into the 20th century.

In 1904, a millenarian movement, similar to that of the Ghost Dance in the North American West, erupted among the Mocovis of San Javier, Santa Fe, Argentina, but was quickly squelched when 500 of them were repulsed after an attack on the town.