Ouranosaurus was a relatively large euornithopod. Taquet in 1976 estimated the body length at 7 metres (23 feet), the weight at 4 tonnes (4.4 short tons). Gregory S. Paul in 2010 gave a higher length estimate of 8.3 metres (27 feet) but a lower weight of 2.2 t (2.4 short tons), emphasizing that the animal was relatively light-built. The femur is 811 millimetres (2.661 ft) long.
In January 1965 Philippe Taquet discovered dinosaurian fossils at the Camp des deux Arbres site near Gadoufaoua. The material was recovered in 1966. Taquet described the type species Ouranosaurus nigeriensis from the fossils in 1976. The generic name is derived from Tuareg ourane meaning “monitor lizard” — a totem animal to the Tuareg who consider it their ancestral maternal uncle — but itself related to Arab waran, “brave”. The specific name refers to Niger.
The holotype specimen MNHN GDF 300, was found in the Upper Elrhaz Formation dating to the Aptian, between 125 and 112 million years old. It consists of an almost complete skeleton with skull, that is today mounted in the Nigerien capital Niamey; the Museum national d’histoire naturelle displays a cast. Other finds include the paratype specimen GDF 381, a second skeleton found in 1972, and the referred specimens GDF 301, a large coracoid, and GDF 302, a femur.
Taquet originally assigned Ouranosaurus to the Iguanodontidae, within the larger Iguanodontia. However, although it shares some similarities with Iguanodon (such as a thumb spike), Ouranosaurus is no longer placed in the iguanodontid family, a grouping that is now considered paraphyletic, a series of subsequent offshoots from the main stem-line of iguandontian evolution. It is instead placed in the clade Hadrosauroidea, which contains the Hadrosauridae (also known as “duck-billed dinosaurs”) and their closest relatives. Ouranosaurus appears to represent an early specialised branch in this group, showing in some traits independent convergence with the hadrosaurids. It is thus a basal hadrosauroid.