Iguazú Falls or Iguaçu Falls (Guarani: Chororõ YguasuSpanish: Cataratas del Iguazú ; Portuguese: Cataratas do Iguaçu ) are waterfalls of the Iguazu River on the border of the Argentine province of Misiones and the Brazilian state of Paraná. Together, they make up the largest waterfall system in the world. The falls divide the river into the upper and lower Iguazu. The Iguazu River rises near the heart of the city of Curitiba. For most of its course, the river flows through Brazil; however, most of the falls are on the Argentine side. Below its confluence with the San Antonio River, the Iguazu River forms the border between Argentina and Brazil.

The name Iguazú comes from the Guarani or Tupi words "y"meaning "water", and "ûasú"meaning "big". Legend has it that a deity planned to marry a beautiful woman named Naipí, who fled with her mortal lover Tarobá in a canoe. In a rage, the deity sliced the river, creating the waterfalls and condemning the lovers to an eternal fall. The first European to record the existence of the falls was the Spanish Conquistador Álvar Núñez Cabeza de Vaca in 1541. It was inscribed into the UNESCO World Heritage List in 2013.

The staircase character of the falls consists of a two-step waterfall formed by three layers of basalt. The steps are 35 and 40 metres (115 and 131 ft) in height. The columnar basalt rock sequences are part of the 1,000-metre-thick (3,300 ft) Serra Geral formation within the Paleozoic-Mesozoic Paraná Basin. The tops of these sequences are characterized by 8–10 m (26–33 ft) of highly resistant vesicular basalt and the contact between these layers controls the shape of the falls. Headwater erosion rates are estimated at 1.4–2.1 cm/year (0.55–0.83 in/year). Numerous islands along the 2.7-kilometre-long (1.7 mi) edge divide the falls into many separate waterfalls and cataracts, varying between 60 and 82 m (197 and 269 ft) high. The number of these smaller waterfalls fluctuates from 150 to 300, depending on the water level. About half of the river's flow falls into a long and narrow chasm called the Devil's Throat (Garganta del Diablo in Spanish or Garganta do Diabo in Portuguese).