Gryposaurus (meaning "hooked-nosed (Greek grypos) lizard"; sometimes incorrectly translated as "griffin (Latin gryphus) lizard") was a genus of duckbilled dinosaur that lived about 80 to 75 million years ago, in the Late Cretaceous (late Santonian to late Campanian stages) of North America. Named species of Gryposaurus are known from the Dinosaur Park Formation in Alberta, Canada, and two formations in the United States: the Lower Two Medicine Formation in Montana and the Kaiparowits Formation of Utah. A possible additional species from the Javelina Formation in Texas may extend the temporal range of the genus to 66 million years ago.

Gryposaurus is similar to Kritosaurus, and for many years the two were thought to be synonyms. It is known from numerous skulls, some skeletons, and even some skin impressions that show it to have had pyramidal scales projecting along the midline of the back. It is most easily distinguished from other duckbills by its narrow arching nasal hump, sometimes described as similar to a "Roman nose," and which may have been used for species or sexual identification, and/or combat with individuals of the same species. A large bipedal/quadrupedal herbivore around 8 meters (26 feet) long, it may have preferred river settings.

Gryposaurus is based on specimen NMC 2278, a skull and partial skeleton collected in 1913 by George F. Sternberg from what is now known as the Dinosaur Park Formation of Alberta, along the Red Deer River. This specimen was described and named by Lawrence Lambe shortly thereafter, Lambe drawing attention to its unusual nasal crest. A few years earlier, Barnum Brown had collected and described a partial skull from New Mexico, which he named Kritosaurus. This skull was missing the snout, which had eroded into fragments; Brown restored it after the duckbill now known as Edmontosaurus annectens, which was flat-headed, and believed that some unusual pieces were evidence of compression. Lambe's description of Gryposaurus provided evidence of a different type of skull configuration, and by 1916 the Kritosaurus skull had been redone with a nasal arch and both Brown and Charles Gilmore had proposed that Gryposaurus and Kritosaurus were one and the same. This idea was reflected in William Parks's naming of a nearly complete skeleton from the Dinosaur Park Formation as Kritosaurus incurvimanus, not Gryposaurus incurvimanus (although he left Gryposaurus notabilis in its own genus). Direct comparison between Kritosaurus incurvimanus and Gryposaurus notabilis is hindered by the fact that the incurvimanus type specimen is missing the front part of the skull, so the full shape of the nasal arch cannot be seen. The 1942 publication of the influential Lull and Wright monograph on hadrosaurs sealed the Kritosaurus/Gryposaurus question for nearly fifty years in favor of Kritosaurus. Reviews beginning in the 1990s, however, called into question the identity of Kritosaurus navajovius, which has limited material for comparison with other duckbills. Thus, Gryposaurus has once again been separated, at least temporarily, from Kritosaurus.

This situation is made more confusing by old suggestions by some authors, including Jack Horner, that Hadrosaurus is also the same as either Gryposaurus, Kritosaurus, or both. This hypothesis was most common in the late 1970s–early 1980s, and appears in some popular books; one well-known work, The Illustrated Encyclopedia of Dinosaurs, uses Kritosaurus for the Canadian material (Gryposaurus), but identifies the mounted skeleton of K. incurvimanus as Hadrosaurus in a photo caption. Although Horner in 1979 used the new combination Hadrosaurus [Kritosaurus] notabilis for a partial skull and skeleton and a second less-complete skeleton from the Bearpaw Shale of Montana (which have since fallen out of the literature), by 1990 he had changed his position, and was among the first to again use Gryposaurus in print. Current thought is that Hadrosaurus, although known from fragmentary material, can be distinguished from Gryposaurus by differences in the upper arm and ilium.

Further research has revealed the presence of a second species, G. latidens, from slightly older rocks in Montana than the classic gryposaur localities of Alberta. Based on two parts of a skeleton collected in 1916 for the American Museum of Natural History, G. latidens is also known from bonebed material. Horner, who described the specimens, considered it to be a less derived species.

New material from the Kaiparowits Formation of Utah, in Grand Staircase–Escalante National Monument, includes a skull and partial skeleton that represent the species G. monumentensis. Its skull was more robust than that of the other species, and its predentary had enlarged prongs along its upper margin, where the lower jaw's beak was based. This new species greatly expands the geographic range of this genus, and there may be a second, more lightly built species present as well. Multiple gryposaur species are known from the Kaiparowits Formation, represented by cranial and postcranial remains, and were larger than their northern counterparts.

In Texas, specifically at the Javelina Formation and the El Picacho Formation, indeterminate hadrosaur remains resembling Kritosaurus and Gryposaurus have been unearthed for decades, but none were considered to be identifiable as a determined genus of hadrosaur, but do resemble some species of Kritosaurini or at least some species of Kritosaurus. However, in 2016, a possibly forth valid species of Gryposaurus named G. alsatei, which was named after Alsate, who was the last leader of the Mescalero Apaches, was unearthed in the Javelina Formation in Texas. Further research is needed to confirm its validity.

As of 2016, there are currently three named species that are recognized as valid today: G. notabilis, G. latidens, and G. monumentensis. The type species G. notabilis is from the late Campanian-age Upper Cretaceous Dinosaur Park Formation of Alberta, Canada. It is now thought that another species from the same formation, Kritosaurus incurvimanus (also known as Gryposaurus incurvimanus), is a synonym of G. notabilis. The two had been differentiated by the size of the nasal arch (larger and closer to the eyes in G. notabilis) and the form of the upper arm (longer and more robust in K. incurvimanus). Ten complete skulls and twelve fragmentary skulls are known for G. notabilis along with postcrania, as well as with two skeletons with skulls that had been assigned to K. incurvimanus. G. latidens, from the late Santonian-early Campanian Lower Two Medicine Formation of Pondera County, Montana, USA, is known from partial skulls and skeletons from several individuals. Its nasal arch is prominent like that of G. notabilis, but farther forward on the snout, and its teeth are less derived, reflecting iguanodont-like characteristics. The informal name "Hadrosauravus" is an early, unused name for this species. G. monumentensis is known from a skull and partial skeleton from Utah. G. monumentensis was listed second on the top 10 list of new species in 2008 by the International Institute for Species Exploration. Recently, a possible fourth species of Gryposaurus, Gryposaurus alsatei, was unearthed in the Javelina Formation, which dates to the late Maastrichtian, along with an unnamed species of Kritosaurus and an undescribed saurolophine which closely resembles Saurolophus, but with a more solid crest. Fossil remains of Gryposaurus have also been unearthed in the El Picacho Formation in Texas.