Tenontosaurus (/tɪˌnɒntəˈsɔːrəs/ ti-NON-tə-SOR-əs; lit. 'sinew lizard') is a genus of iguanodontian ornithopod dinosaur. It had an unusually long, broad tail, which like its back was stiffened with a network of bony tendons. The genus is known from the late Aptian to Albian ages of the Early Cretaceous period sediments of western North America, dating between 115 and 108 million years ago. It contains two species, Tenontosaurus tilletti (described by John Ostrom in 1970) and Tenontosaurus dossi (described by Winkler, Murry, and Jacobs in 1997). Many specimens of T. tilletti have been collected from several geological formations throughout western North America. T. dossi is known from only a handful of specimens collected from the Twin Mountains Formation of Parker County, Texas.

The first Tenontosaurus fossil was found in Big Horn County, Montana by an American Museum of Natural History (AMNH) expedition in 1903. Subsequent digs in the same area during the 1930s, spearheaded by Barnum Brown and Roland T. Bird, unearthed 18 more specimens. Four more were recovered during the 1940s: two by a team from the University of Oklahoma, and two by a private collector, Al Silberling, operating on the behalf of Princeton University. Despite the large number of fossil specimens, the animal was not named or scientifically described during this time, though Barnum Brown gave it the informal name "Tenantosaurus", "sinew lizard", in reference to the extensive system of stiffening tendons in its back and tail.

Starting in 1962, Yale University conducted an extensive, five-year-long dig in the Big Horn Basin area (Cloverly Formation) of Montana and Wyoming. The expedition was led by John Ostrom, whose team discovered more than 40 new specimens of the taxon recovered by Brown. Following his expedition, in 1970, Ostrom published a review of the fauna and geology of the Cloverly Formation. In that paper, he scientifically described Brown's taxon, calling it Tenontosaurus tilletti. The genus name has the same etymology as the informal "Tenantosaurus" name; the species name refers to the Lloyd Tillett family, who provided field parties from Yale with assistance and hospitality throughout their expeditions. The type specimen of T. tilletti, as designated by Ostrom, was AMNH 3040.

Since 1970, many more Tenontosaurus specimens have been reported, both from the Cloverly and other geological formations, including the Antlers Formation in Oklahoma, Paluxy Formation of Texas, Wayan Formation of Idaho, Cedar Mountain Formation of Utah, and Arundel Formation of Maryland. In 1997, remains from the Twin Mountains Formation of Texas were assigned to a new species of Tenontosaurus, T. dossi, named after the Doss Ranch site.

Tenontosaurus was a medium-sized ornithopod, with both species weighing about 1,000 kilograms (2,200 lb). Gregory S. Paul in 2016 estimated that T. tilletti would have been 6 metres (20 ft) and weighed 600 kilograms (1,300 lb), but Nicolás E. Campione and David C. Evans in 2020 estimated that this species would have weighed up to 971–1,019 kilograms (2,141–2,247 lb). Paul also estimated that T. dossi would have been 7 meters (23 ft) long and weighed 1,000 kilograms (2,200 lb). The original describers of T. dossi favoured a length estimate of 7 and 8 metres (23 and 26 ft), and the same estimate has been given elsewhere.

The skull of Tenontosaurus was originally described by Ostrom as being very long and deep compared to taxa such as Theiophytalia (then considered a species of Camptosaurus). While this is true, Ostrom's reconstruction of the skull exaggerated the overall depth of the skull, leading to one which more closely resembles that of Hypsilophodon. The external nares (nostril openings) of Tenontosaurus were very large, and were almost entirely encircled by the premaxillae. The premaxillae flared inferiorly (towards the bottom), forming a thick, U-shaped beak, characteristic of iguanodontians. The beak extended as far back as the anterior (front) process of the maxilla. The single longest bone in Tenontosaurus' skull is the vomer, which stretches from the middle of the beak to the very back of the orbital cavity (eye socket). The maxilla was larger in general, though, comprising the majority of the rostrum and housing all of the upper teeth. The maxilla was divided into two components: a facial lamina, which comprised its lateral (outer) surface, and a dental ridge. In more derived iguanodontians, and Theiophytalia, the maxilla was bordered by a lateral process of the jugal relatively far forward; in Tenontosaurus, however, the facial laminae of the maxillas overlapped with the jugals, and almost contacted the inferior part of the lacrimal bone. T. tilletti had a long and narrow antorbital fenestra, which extended down from the lacrimals to the facial laminae; T. dossi, meanwhile, had a relatively small one, surrounded by a shallow fossa. Similarly, the dorsal (upper) process of T. dossi's maxilla did not expand posteriorly above the antorbital fenestra, as it did in T. tilletti. The process connecting the quadratojugal to the jugal was structured in a way that formed an additional temporal fenestra, which Ostrom presumed related to jaw adductor muscles. A similar fenestra is also seen in Hypsilophodon. The orbit was roughly triangular, and was larger than the fenestrae anterior to (in front of) or posterior to (behind) it. The majority of the occipital condyle in Tenontosaurus was made up of a large basioccipital, though this structure was almost entirely excluded from the occipital surface itself. Ostrom noted similarities between the shapes of the paroccipitals in Tenontosaurus and in hadrosaurs, namely their hooked shape and their orientation in relation to the foramen magnum. The ventral flange of the pterygoid was broad and deep, particularly enlarged dorsally, providing a larger attachment site for parts of the pterygoid muscles. The quadrate was narrow, and was essentially vertical.

Tenontosaurus' mandible (lower jaw) was described by Ostrom as being "of moderate length and robustness". It terminated in a predentary bone, a structure found uniquely among ornithischians which formed the lower beak. The predentary in Tenontosaurus was horseshoe-shaped and was lined with projecting serrations, referred to as pseudo-teeth, or as denticles. They likely served, at least partly, as anchors for the keratinous portion of the beak. Such structures are present in many ornithischians, save for taxa such as Dryosaurus or Zalmoxes. In at least T. dossi, the predentary had a process on its ventral (bottom) surface which overlapped with the mandibular symphyses. The dentaries (the tooth-bearing portions of the lower jaw) were robust. There were two distinct coronoid processes to the mandibles. The retroarticular processes, projections at the back of each mandibular ramus, were long and curved. Ostrom noted that the surangulars likely formed most of the lateral surface of the mandible at the level of the coronoid process and beyond, though was not able to confirm this due to the condition of the material at his disposal; this was later confirmed. The surangulars also formed the lateral part of the retoarticular processes.

All of Tenontosaurus' teeth were enameled unilaterally (on only one side, that being medially, or on the inside). The maxillary teeth bore a series of ridges, subequal in length, running along their surface in a non-parallel arrangement. Those of the dentary lacked these ridges, instead bearing prominent vertical keels on their inner surfaces.