Megatherium (/mɛɡəˈθɪəriəm/ meg-ə-THEER-ee-əm; from Greek méga (μέγα) 'great' + theríon (θηρίον) 'beast') is an extinct genus of large ground sloths endemic to South America that lived from the Early Pliocene through the end of the Late Pleistocene. It is best known for the elephant-sized, 3.5–4 tonnes (7,700–8,800 lb) type species Megatherium americanum, primarily known from the Pampas, but ranging southwards to northernmost Patagonia and northwards to southern Bolivia during the late Middle Pleistocene and Late Pleistocene. Various other species have been described, including those belonging to the Andean subgenus Pseudomegatherium, which range in size from comparable to M. americanum down to approximately 1 tonne (2,200 lb).

The first (holotype) specimen of Megatherium americanum was discovered in 1787 on the bank of the Luján River in what is now northern Argentina. The specimen was then shipped to Spain the following year wherein it caught the attention of the pioneering French paleontologist Georges Cuvier, who named the animal in 1796, making it one of the first prehistoric animals to be scientifically named, and was the first to determine, by means of comparative anatomy, that Megatherium was a giant sloth.

Megatherium is part of the sloth family Megatheriidae, which also includes the closely related and similarly giant Eremotherium, comparable in size to M. americanum, which was native to tropical South America, Central America and North America as far north as the southern United States.

Megatherium americanum is thought to have been a browser that fed on the foliage and twigs of trees and shrubs using a black rhinoceros–like prehensile upper lip (rather than a prehensile tongue as often historically depicted). Despite its large body size, Megatherium americanum is widely thought to have been able to rear up on its hind legs at least while standing and braced, which allowed it to grasp and feed on high-growing leaves, as well as possibly to use its claws for defense.

Megatherium became extinct around 12,000 years ago as part of the end-Pleistocene extinction event, simultaneously with the majority of other large mammals in the Americas. The extinctions followed the first arrival of humans in the Americas, and at least one and potentially several sites where M. americanum was slaughtered and butchered by humans are known, suggesting that hunting could have been a factor in its extinction.

The earliest specimen of Megatherium americanum was discovered in 1787 by Manuel de Torres, a Dominican friar and naturalist, from a ravine on the banks of the Lujan River in what is now northern Argentina, which at the time was part of the Viceroyalty of the Río de la Plata in the Spanish Empire. Torres described the bones as a 'wonder and providence of the Lord'. On the orders of the then viceroy of la Plata, Nicolás Cristóbal del Campo, Marqués de Loreto, the specimen was moved to the capital Buenos Aires. There the skeleton was drawn for the first time by José Custodio Sáa y Faria in a horse-like posture. Campo summoned a number of local indigenous leaders to ask if they had heard of the animal. The skeleton was then transferred by Campo to the Royal Cabinet of Natural History of Madrid (now the National Museum of Natural Sciences MNCN) in 7 crates, which had arrived and been unpacked by late 1788.

At the direction of the cabinets main taxidermist Juan Bautista Bru, the specimen was then mounted for public exhibition (The specimen is still on display in the National Museum of Natural Sciences today, with the mount still unaltered from Bru's original effort). In 1796 a scientific description of the skeleton was published authored by Bru along with engineer Joseph Garriga, with engravings by Manuel Navarro. As the work was going through the process of publication in 1795, preliminary prints of the paper were obtained by French diplomat Philippe-Rose Roume who was in Madrid at the time, who sent them to the National Museum of Natural History (Muséum national d'histoire naturelle) in Paris, France, where they were seen by French anatomist and paleontologist Georges Cuvier.

Cuvier, working solely from the prints from Madrid and not visiting the specimen personally, and using comparative anatomy with "edentate" mammals (now recognised as members of the order Xenarthra) in the collection of the Paris museum, correctly recognised that the remains represented those of a giant sloth, and an animal that was entirely extinct and not living. In early 1796, somewhat before the full publication of the work by Bru, Garriga and Navarro, Cuvier published a paper naming the species Megatherium americanum (literally "Great American beast"), becoming the first fossil mammal to be identified with both a genus and species name. Which description had priority has been controversial in the past. Cuvier later wrote a fuller description in 1804, which was republished in his famous 1812 book Recherches sur les ossemens fossiles de quadrupèdes. Cuvier identified Megatherium as a sloth primarily on the basis of its skull morphology, the dental formula and the shoulder, while regarding the anatomy of its limbs as more similar to armadillos and anteaters. Cuvier suggested that based on the proportions of its limbs (which are approximately equal to each other), that Megatherium did not jump or run, nor crawl like living sloths, with the presence of a clavicle and well developed crests on the humerus, suggesting to Cuvier that the animal probably used its forelimbs to grasp. A later publication in 1823 by Cuvier suggested that giant carapaces found in the Pampas also belonged to Megatherium, but British paleontologist Richard Owen in 1839 demonstrated that these actually belonged to another extinct group of xenarthrans called glyptodonts that were related to armadillos.