Ankylosaurus is a genus of armored dinosaur. Its fossils have been found in geological formations dating to the very end of the Cretaceous Period, about 68–66 million years ago, in western North America, making it among the last of the non-avian dinosaurs. It was named by Barnum Brown in 1908; it is monotypic, containing only A. magniventris. The generic name means "fused" or "bent lizard", and the specific name means "great belly". A handful of specimens have been excavated to date, but a complete skeleton has not been discovered. Though other members of Ankylosauria are represented by more extensive fossil material, Ankylosaurus is often considered the archetypal member of its group, despite having some unusual features.

Possibly the largest known ankylosaurid, Ankylosaurus is estimated to have been between 6 and 8 m (20 and 26 ft) long and to have weighed between 4.8 and 8 t (5.3 and 8.8 short tons). It was quadrupedal, with a broad, robust body. It had a wide, low skull, with two horns pointing backward from the back of the head, and two horns below these that pointed backward and down. Unlike other ankylosaurs, its nostrils faced sideways rather than towards the front. The front part of the jaws was covered in a beak, with rows of small, leaf-shaped teeth farther behind it. It was covered in armor plates, or osteoderms, with bony half-rings covering the neck, and had a large club on the end of its tail. Bones in the skull and other parts of the body were fused, increasing their strength, and this feature is the source of the genus name.

Ankylosaurus is a member of the family Ankylosauridae, and its closest relatives appear to be Anodontosaurus and Euoplocephalus. Ankylosaurus is thought to have been a slow-moving animal, able to make quick movements when necessary. Its broad muzzle indicates it was a non-selective browser. Sinuses and nasal chambers in the snout may have been for heat and water balance or may have played a role in vocalization. The tail club is thought to have been used in defense against predators or in intraspecific combat. Specimens of Ankylosaurus have been found in the Hell Creek, Lance, Scollard, Frenchman, and Ferris formations, but it appears to have been rare in its environment. Although it lived alongside Denversaurus, a nodosaurid ankylosaur, their ranges and ecological niches do not appear to have overlapped, and Ankylosaurus may have inhabited upland areas. Ankylosaurus also lived alongside dinosaurs such as Tyrannosaurus, Triceratops, and Edmontosaurus.

In 1906, an American Museum of Natural History expedition led by American paleontologist Barnum Brown discovered the type specimen of Ankylosaurus magniventris (AMNH 5895) in the Hell Creek Formation, near Gilbert Creek, Montana. The specimen (found by collector Peter Kaisen) consisted of the upper part of a skull, two teeth, part of the shoulder girdle, cervical, dorsal, and caudal vertebrae, ribs, and more than thirty osteoderms (armor plates). Brown scientifically described the animal in 1908; the generic name is derived from the Greek words αγκυλος ankulos ('bent' or 'crooked'), referring to the medical term ankylosis, the stiffness produced by the fusion of bones in the skull and body, and σαυρος sauros ('lizard'). The name can be translated as "fused lizard", "stiff lizard", or "curved lizard". The type species name, magniventris, is derived from the Latin: magnus ('great') and Latin: venter ('belly'), referring to the great width of the animal's body.

The skeletal reconstruction accompanying the 1908 description restored the missing parts in a fashion similar to Stegosaurus, and Brown likened the result to the extinct armored mammal Glyptodon. In contrast to modern depictions, Brown's stegosaur-like reconstruction showed robust forelimbs, a strongly arched back, a pelvis with prongs projecting forwards from the ilium and pubis, as well as a short, drooping tail without a tail club, which was unknown at the time. Brown also reconstructed the armor plates in parallel rows running down the back; this arrangement was purely hypothetical. Brown's reconstruction became highly influential, and restorations of the animal based on his diagram were published as late as the 1980s. In a 1908 review of Brown's Ankylosaurus description, the American paleontologist Samuel Wendell Williston criticized the skeletal reconstruction as being based on too few remains, and claimed that Ankylosaurus was merely a synonym of the genus Stegopelta, which Williston had named in 1905. Williston also stated that a skeletal reconstruction of the related Polacanthus by Hungarian paleontologist Franz Nopcsa was a better example of how ankylosaurs would have appeared in life. The claim of synonymy was not accepted by other researchers, and the two genera are now considered distinct.

Brown had collected 77 osteoderms while excavating a Tyrannosaurus specimen in the Lance Formation of Wyoming in 1900. He mentioned these osteoderms (specimen AMNH 5866) in his description of Ankylosaurus but thought they belonged to the Tyrannosaurus instead. Paleontologist Henry Fairfield Osborn also expressed this view when he described the Tyrannosaurus specimen as the now synonymous genus Dynamosaurus in 1905. More recent examination has shown them to be similar to those of Ankylosaurus; it seems that Brown had compared them with some Euoplocephalus osteoderms, which had been erroneously cataloged as belonging to Ankylosaurus at the AMNH.

In 1910, another AMNH expedition led by Brown discovered an Ankylosaurus specimen (AMNH 5214) in the Scollard Formation by the Red Deer River in Alberta, Canada. This specimen included a complete skull, mandibles, the first and only tail club known of this genus, as well as ribs, vertebrae, limb bones, and armor. In 1947 the American fossil collectors Charles M. Sternberg and T. Potter Chamney collected a skull and mandible (specimen CMN 8880, formerly NMC 8880), 1 kilometer (5⁄8 mile) north of where the 1910 specimen was found. This is the largest-known Ankylosaurus skull, but it is damaged in places. A section of caudal vertebrae (specimen CCM V03) was discovered in the 1960s in the Powder River drainage, Montana, part of the Hell Creek Formation. In addition to these five incomplete specimens, many other isolated osteoderms and teeth have been found.

In 1990, American paleontologist Walter P. Coombs pointed out that the teeth of two skulls assigned to A. magniventris differed from those of the holotype specimen in some details, and though he expressed a "considerate temptation" to name a new species of Ankylosaurus for these, he refrained from doing so, as the range of variation in the species was not completely documented. He also raised the possibility that the two teeth associated with the holotype specimen perhaps did not belong to it, as they were found in matrix within the nasal chambers. The American paleontologist Kenneth Carpenter accepted the teeth as belonging to A. magniventris in 2004, and that all the specimens belonged to the same species, noting that the teeth of other ankylosaurs are highly variable.