Valdosaurus ("Weald lizard") is a genus of bipedal herbivorous dryosaurid dinosaur known from the Early Cretaceous of the Isle of Wight and elsewhere in England. Originally its remains were believed to belong to Hypsilophodon. In 1889, Richard Lydekker assigned them to Camptosaurus, alongside a partial lower jaw, and applied the name Camptosaurus valdensis. In the 1970s, Peter Galton reassigned these remains to a new species of Dryosaurus, then to an entirely new genus and species, that being Valdosaurus proper. The type and only species of Valdosaurus, named by Galton, is V. canaliculatus.

Numerous specimens have been assigned to Valdosaurus over the years. Some, namely those found in Niger (now Elrhazosaurus), have been reassigned to different genera, while others have simply been redesignated as indeterminate. With that said, additional specimens of V. canaliculatus have been discovered. The most complete is a specimen discovered in 2012 at Compton Bay on the Isle of Wight, which consists of a largely articulated rear half of the animal. Whereas the type specimens were very small, with a femur (thigh bone) length of just 14 cm (0.46 ft) and an estimated body mass of 10 kg (22 lb), larger specimens such as the Compton specimen may have reached body lengths of around 4 m (13 ft) and body masses of roughly 400 kg (880 lb).

At some point in the 19th century, the Rev. William Fox collected two small femora (upper leg bones) near Cowleaze Chine on the south-west coast of the Isle of Wight. In 1888, these specimens, BMNH R184 and R185, were assigned to Hypsilophodon foxii by Richard Lydekker. The femora were associated with a tibia, and were subsequently assigned to the possible tyrannosauroid genus Calamosaurus. In 1889, Lydekker assigned them to Camptosaurus, alongside a partial mandibular ramus (half of a lower jaw), BMNH R180, applying the species name C. valdensis to all of these remains. The species name, valdensis derives from valdo, a modification of the Old English word weald ("a wood") which refers to the Weald area.

In a 1974 paper, Peter Galton examined the holotype femur of C. valdensis, and came to the conclusion that it belonged to a large specimen of Hypsilophodon; he believed, as had been suggested by Sir Richard Owen more than a century prior, that BMNH R180 belonged to a juvenile Iguanodon. In 1975, Galton re-examined the femora, and concluded that they did not belong to Camptosaurus. He therefore assigned them to Dryosaurus, and applied the name Dryosaurus canaliculatus. The specific name means "with a small channel" in Latin, referring to a distinct groove between the condyles of the lower femora. Two years later, Galton recognised that they were in fact morphologically distinct from Dryosaurus, and erected a new genus for them, Valdosaurus. The type species, D. canaliculatus, was thus renamed V. canaliculatus. Galton speculated that Valdosaurus might have been a direct descendant of Dryosaurus, based on the supposed ubiquity of that genus across Europe, Africa, and Asia. Furthermore, Galton reassigned BMNH R180 to Valdosaurus, after briefly and provisionally considering it a specimen of Vectisaurus.

A second species, V. nigeriensis, was described by Galton and Philippe Taquet from younger rocks from Niger in 1982. In 1998 William Blows inadvertently named another species, Valdosaurus dextrapoda, by including this name in a fauna list. This was an error, and the species has never been supported. In 2009, Galton reviewed the Valdosaurus material. He concluded that no fossils from outside England could be reliably referred to the genus. He thus gave V. nigeriensis its own genus: Elrhazosaurus. Even where many of the English specimens were concerned it was uncertain whether they belonged to Valdosaurus, including all cranial elements and teeth. Some hindlimb and pelvic bones from the Upper Weald Clay Formation (late Barremian) were referable to V. canaliculatus. Some material from the earlier Hastings Beds (Valanginian) were referred to a Valdosaurus sp. Galton established that Richard Owen had in 1842 been the first to describe Valdosaurus thighbones, specimens BMB 004297-004300, assigning them to Iguanodon.

Having a close European relative of the American form Dryosaurus named led to most of the dryosaurid fossil material of Europe being referred to Valdosaurus. Valdosaurus was seen as not only present in England (the Wessex Formation of the Isle of Wight and the Hastings Beds of West Sussex). Records from Spain remain questionable. These rock units were deposited between the Berriasian and Barremian stages, between approximately 145 and 125 million years ago. V. canaliculatus would then be known from thigh bones, extensive additional postcranial elements, partial lower jaws, and teeth.

In 1888, Richard Lydekker described an ilium, BMNH R2150 from Sussex, assigning it to Hylaeosaurus. That specimen was subsequently assigned to Valdosaurus canaliculatus. In 2012, a new specimen of Valdosaurus was discovered, in Compton Bay on the Isle of Wight, by Nick Chase; the exact location of its discovery has been withheld to prevent attempts to steal material from the site. The specimen, since designated IWCMS 2013.175, is the most complete Valdosaurus yet found. It was found partly articulated and includes a partial dorsal (back) series, an almost complete caudal (tail) series, pelvic material, and both hind limbs. In life, the specimen would have been around 4 m (13 ft) long.

In his 2009 re-examination of Valdosaurus, Peter Galton emphasised that though the type femora were very small, 14 cm (0.46 ft) in length, which has led to estimates of a length of 1.2 m (3.9 ft) and a weight of 10 kg (22 lb), they came from a juvenile individual; an adult would have been a "medium-sized euornithopod", with femora reaching a length of something like 50 cm (1.6 ft), a length attained by specimen MIWG.6879 (a complete hind limb). In 2001, Darren Naish and David Martill suggested based on that specimen that skeletally mature V. canaliculatus would have a maximum body length of about 4 m (13 ft), a shoulder height of about 1.4 m (4.6 ft), and a body mass of 400 kg (880 lb), though most specimens were about 3.5 m (11 ft) in length.