Archaeoindris fontoynontii is an extinct, giant lemur and the largest primate known to have evolved on Madagascar, comparable in size to a male gorilla. It belonged to a family of extinct lemurs known as "sloth lemurs" (Palaeopropithecidae), and because of its extremely large size, it has been compared to the extinct ground sloths of North and South America. It was most closely related to Palaeopropithecus, the second largest type of sloth lemur. Along with the other extinct sloth lemurs, Archaeoindris was related to the living indri, sifakas, and woolly lemurs, as well as the recently extinct monkey lemurs (Archaeolemuridae). It probably became extinct recently, around 350 BCE.
Its remains have been found at only one location: Ampasambazimba, a subfossil site in central Madagascar. The skeleton of Archaeoindris was massive and robust, and shared many traits with that of Palaeopropithecus. The arms were longer than the legs, but no hand or foot bones have been found for comparison with the other sloth lemurs. Size estimates based on the limited remains have varied widely, ranging as high as 244.1 kg (538 lb), but the most thorough statistical investigation estimates a mass of 160 kg (350 lb). There are varying opinions about the way Archaeoindris moved in its environment, ranging from tree-dwelling to ground-dwelling. Its skeleton suggests it was a deliberate climber that visited the ground to travel. The diet of Archaeoindris was mostly leaves, and its habitat—prior to human arrival—was a mix of woodlands, bushlands, and savanna, rich in lemur diversity. Although it was a rare lemur, it was still extant when humans first arrived on Madagascar, and it would have been vulnerable to hunting and habitat loss.