Edmontosaurus (/ɛdˌmɒntəˈsɔːrəs/ ed-MON-tə-SOR-əs) (meaning "lizard from Edmonton"), often colloquially and historically known as Anatosaurus or Anatotitan (meaning "duck lizard" and "giant duck"), is a genus of hadrosaurid (duck-billed) dinosaur. It contains two known species: Edmontosaurus regalis and Edmontosaurus annectens. Fossils of E. regalis have been found in rocks of western North America that date from the late Campanian age of the Cretaceous period 73 million years ago, while those of E. annectens were found in the same geographic region from rocks dated to the end of the Maastrichtian age, 66 million years ago. Edmontosaurus was one of the last non-avian dinosaurs ever to exist, and lived alongside dinosaurs like Triceratops, Tyrannosaurus, Ankylosaurus, and Pachycephalosaurus shortly before the Cretaceous–Paleogene extinction event.

Edmontosaurus included two of the largest hadrosaurid species, with E. annectens measuring up to 12 metres (39 ft) in length and weighing around 5.6 metric tons (6.2 short tons) in average asymptotic body mass. The exceptionally large specimens of E. annectens measured around 15 metres (49 ft) long and weighed around 15.9 metric tons (17.5 short tons). Several well-preserved specimens are known that include numerous bones, as well as extensive skin impressions and possible gut contents. Edmontosaurus is classified as a genus of saurolophine (or hadrosaurine) hadrosaurid, a member of the group of hadrosaurids that lacked large, hollow crests and instead had smaller, solid crests or fleshy combs.

The first fossils named Edmontosaurus were discovered in southern Alberta (named after Edmonton, the capital city), in the Horseshoe Canyon Formation (formerly called the lower Edmonton Formation). The type species, E. regalis, was named by Lawrence Lambe in 1917, although several other species that are now classified in Edmontosaurus were named earlier. The best known of these is E. annectens, named by Othniel Charles Marsh in 1892. This species was originally known as a species of Claosaurus, known for many years as a species of Trachodon, and later known as Anatosaurus annectens. Anatosaurus, Anatotitan, and probably Ugrunaaluk are now generally regarded as synonyms of Edmontosaurus.

Edmontosaurus was widely distributed across western North America, ranging from Colorado to the northern slopes of Alaska. The distribution of Edmontosaurus fossils suggests that it preferred coasts and coastal plains. It was a herbivore that could move on both two legs and four. Because it is known from several bone beds, Edmontosaurus is thought to have lived in groups and may have been migratory as well. The wealth of fossils has allowed researchers to study its paleobiology in detail, including its brain, how it may have fed, and its injuries and pathologies, such as evidence for tyrannosaur attacks on a few specimens.

In an Edmontosaurus fossil, Tuinstra et al. (2025) made the first clear detection of actual original dinosaur organic material, showing the presence of ancient hydroxyproline (a building block of collagen) in the fossil, and refuting the hypothesis that organic matter present in fossils must be due to contamination.

Edmontosaurus has had a very long and complicated history in paleontology, having spent decades with various species classified in other genera. Its taxonomic history intertwines at various points with the genera Agathaumas, Anatosaurus, Anatotitan, Claosaurus, Hadrosaurus, Thespesius, and Trachodon, with references predating the 1980s typically using Anatosaurus, Claosaurus, Thespesius, or Trachodon for edmontosaur fossils (excluding those assigned to E. regalis) depending on the author and the date. Although Edmontosaurus was only named in 1917, its oldest well-supported species (E. annectens) was named in 1892 as a species of Claosaurus.

The first well-supported species of Edmontosaurus was named in 1892 as Claosaurus annectens by Othniel Charles Marsh. This species is based on USNM 2414, which is a partial skull-roof and skeleton, with a second skull and skeleton, YPM 2182, designated as the paratype. Both were collected in 1891 by John Bell Hatcher from the late Maastrichtian-age Upper Cretaceous Lance Formation of Niobrara County (then part of Converse County), Wyoming. This species has some historical footnotes attached, as it is among the first dinosaurs to receive a skeletal restoration and is the first hadrosaurid so restored. YPM 2182 and UNSM 2414 are, respectively, the first and second essentially complete mounted dinosaur skeletons in the United States. YPM 2182 was put on display in 1901 and USNM 2414 was put on display in 1904.

Because of the incomplete understanding of hadrosaurids at the time, following Marsh's death in 1897, Claosaurus annectens was variously classified as a species of Claosaurus, Thespesius or Trachodon. Opinions varied greatly, as textbooks and encyclopedias drew a distinction between the "Iguanodon-like" Claosaurus annectens and the "duck-billed" Hadrosaurus (based on remains now known as adult Edmontosaurus annectens), while Hatcher explicitly identified C. annectens as synonymous with the hadrosaurid represented by those same duck-billed skulls. Hatcher's revision, published in 1902, was sweeping, as he considered almost all hadrosaurid genera then known as synonyms of Trachodon. This included Cionodon, Diclonius, Hadrosaurus, Ornithotarsus, Pteropelyx, and Thespesius, as well as Claorhynchus and Polyonax, which are fragmentary genera now thought to be ceratopsians. Hatcher's work led to a brief consensus until post-1910, when new material from Canada and Montana showed a greater diversity of hadrosaurids than previously suspected. Charles W. Gilmore, in 1915, reassessed hadrosaurids and recommended that Thespesius be reintroduced for hadrosaurids from the Lance Formation and rock units of equivalent age and that Trachodon, based on inadequate material, should be restricted to a hadrosaurid from the older Judith River Formation and its equivalents. In regards to Claosaurus annectens, he recommended that it be considered the same as Thespesius occidentalis. His reinstatement of Thespesius for Lance-age hadrosaurids would have other consequences for the taxonomy of Edmontosaurus in the following decades.