Megalneusaurus is a genus of large pliosaurid plesiosaur from the Late Jurassic of North America. It was provisionally described as a species of Cimoliosaurus by the geologist Wilbur Clinton Knight in 1895, before being given its own genus by the same author in 1898. The only known species is M. rex, known from several specimens mainly found in the Redwater Shale Member, within the Sundance Formation, Wyoming, United States. A specimen discovered in the Naknek Formation in southern Alaska was referred to the genus in 1994. The loss of most fossils has led some paleontologists to consider the genus as dubious, although its validity is maintained by many authors. The binominal name literally means "king of large swimming lizards", due to the size of the first specimen.

Estimated to be around 7–9 meters (23–30 ft) long, Megalneusaurus is one of the largest known North American pliosaurs. As its name suggests, the genus was considered the largest sauropterygian identified before the discovery of some Kronosaurus fossils in 1930. Like some other plesiosaurs, Megalneusaurus have four flippers, a short tail, and most likely an elongated head and short neck, suggesting that it is a thalassophonean-like pliosaurid. The rear flippers were larger than those at the front.

The animal lived in the shallow waters of the Sundance Sea, an epicontinental sea covering much of North America during part of the Jurassic. Like other plesiosaurs, Megalneusaurus was well-adapted to aquatic life, using its flippers for a method of swimming known as subaqueous flight. It shared its habitat with invertebrates, fish, ichthyosaurs, and other plesiosaurs, including the cryptoclidids Pantosaurus and Tatenectes. Based on stomach contents, the animal fed on cephalopods and fish, although it could also have fed on contemporary plesiosaurs. The Alaskan specimen also indicates that it would have occupied colder waters, where the fauna was less diverse.

In 1895, geologist Wilbur Clinton Knight went to examine an oil field near the small town of Ervay, Wyoming, USA. The renowned paleontologist Edward Drinker Cope was originally supposed to accompany Knight on this examination, but instead, he sent an individual named Stewart, who worked for him at the American Museum of Natural History. They spent about thirty days together doing their work, but after that, Knight, wanting to go home, left Stewart in another field with other two men. On his way back, while re-examining the oil field, Knight discovered the partially articulated but incomplete fossilized skeleton of a large pliosaur. Lacking the materials to exhume the specimen, he contacted a rancher living nearby who lent him tools and a wagon. The lack of packing materials, however, led Knight to rebury much of the fossil specimen, intending to return to continue his work later. While the geologist was away from the discovery site, Stewart arrived nearby and was quickly informed about the finding by the rancher who had lent his colleague the tools. Stewart then recovered most of the reburied fossils and sent them to the American Museum of Natural History, urging his two colleagues he had previously met to help him with this task. Describing this action as theft, Knight sent two letters in 1898, addressed respectively to the museum director Henry Fairfield Osborn and J. M. Garett, requesting the return of the pliosaur and shark fossils residing in Cope's collection at the museum, which were originally discovered by him.

The fossils of the pliosaur first unearthed by Knight have since been sent to the University of Wyoming, where they are catalogued as UW 4602. These consist of ribs, vertebrae, two more or less partial flippers and part of the shoulder girdle, from an adult specimen. Knight made his discovery official the same year via an announcement that was published by Science, where he briefly described certain fossils. As the fossils were not exhumed from their geological matrix at the time of publication, Knight was uncertain about what genus the specimen pertained to. He therefore named a new species of Cimoliosaurus, C. rex, to which he provisionally classified it pending a more in-depth description. It was three years later, in 1898, that Knight described the specimen in more detail and named the distinct genus Megalneusaurus to include it, the species thus becoming M. rex. The generic name is formed from the Ancient Greek words μέγας (mégas, "great") and νηκτός (nêktós, "swimmer") prefixed onto σαῦρος (saûros, "lizard"), while the specific name rex means "king" in Latin. Combined, the binominal name literally means "king of large swimming lizards". Although no description of the meaning of this etymology was given in Knight's 1898 description, it was named for its size, which was considerable for plesiosaurians then known, which he judged to be "the largest known animal of the order Sauropterygia".

Due to the discovery of other large pliosaurs found elsewhere in the world, Megalneusaurus was mostly ignored by paleontologists, being only very briefly mentioned in 20th and early 21st century scientific literature. Furthermore, additional parts of the holotype including cervical, dorsal and caudal vertebrae, a large part of the shoulder girdle and ribs, have also been lost. It was nevertheless reported in 2003 by the biologist Richard Ellis that the paleontologist Robert Bakker was re-examining the remaining fossil material with the aim of giving an updated and more detailed description. In 1995, the Tate Geological Museum of Casper, Wyoming, planned to create an exhibit showcasing marine reptiles discovered in the Sundance Formation. During the preparations for the exhibit, researchers came across a cast of one the original flippers discovered by Knight. Based on this same casting, American paleontologist William Wahl of the Wyoming Dinosaur Center, began to take a particular interest in this taxon, which led him to open an investigation to find the location of the original type locality of Megalneusaurus. It was during the summer of 1996 that the type locality was finally found, thanks among other things to letters and especially a map drawn by Knight in 1901. The site is located at the Redwater Shale Member of the upper part of the Sundance Formation, which dates to the Oxfordian stage of the Late Jurassic. After this rediscovery, new excavations were launched in the field and additional fossils were found. About 20 m (66 ft) from the area where the first known fossils of Megalneusaurus were discovered, a large bone fragment probably coming from the shoulder or pelvic girdle was exhumed and mentioned in a 2007 article. In 2008 a fully articulated and almost complete forelimb from the holotype specimen was exhumed, and it was described in detail in 2010. From the orientation of this forelimb, it is likely that the humerus, or part of it, was on or near the ground surface and could have been collected in 1895 by either Knight or Stewart. Fossils coming possibly from two other specimens of Megalneusaurus have also been reported. The first is an isolated neural arch (top/dorsal part of the vertebra), cataloged as UW 24238, while the second is a propodial (upper limb bone), cataloged as WDC SS019. Other fossil finds referred to Megalneusaurus were made in this area over an additional period from 2009 until 2011, but these have not yet been officially described.

In addition to the material from Wyoming, a specimen assigned to Megalneusarus is also known from southern Alaska. In 1922, W. R. Smith of the United States Geological Survey received two bone fragments from prospector Jack Mason. These two fragments were collected from the Kejulik River, located on the Alaska Peninsula, and consist of the proximal and distal ends of the same large bone interpreted as a humerus, which was subsequently stored at the National Museum of Natural History, where it is since cataloged as USNM 418489. The stratigraphic unit in which this specimen was discovered corresponds to the Snug Harbor Siltstone Member of the Naknek Formation, dating from between the Oxfordian and Kimmeridgian stages of the Late Jurassic. It was only in 1994 that American paleontologists Robert E. Weems and Robert B. Blodgett described this fossil in detail, referring it to the genus on the basis of comparisons made with the holotype specimen. However, as their description and comparison is only based on an isolated bone, they referred it under the name Megalneusaurus sp., the authors being uncertain as to whether the specimen belonged to M. rex or another species. Although some authors have been doubtful about the attribution of this specimen to the genus in later works, Wahl and colleagues reclassified this specimen as an M. rex in 2010, but considered the bone to be a partial propodial.

Plesiosaurs are usually categorized as belonging to the small-headed, long-necked "plesiosauromorph" morphotype or the large-headed, short-necked "pliosauromorph" morphotype, Megalneusaurus belonging to the latter category. Like all plesiosaurs, it had a short tail, a massive trunk and two pairs of large flippers.