Pliosaurus is the type genus (defining example) of the pliosaurs, one of the major group of the plesiosaurs, an extinct group of aquatic marine reptiles. It lived from the Upper Jurassic to the Lower Cretaceous in what is now Europe. The first known fossil consists of a partial skeleton of an immature specimen collected by William Buckland in Market Rasen, England. Although first mentioned in a 1824 paper by William Daniel Conybeare, it was not until 1841 that it was first described by Richard Owen as belonging to a new species of Plesiosaurus, before being given its own genus by the same author later that year. The genus name means "more lizard", Owen naming the taxon of because its appearance being more reminiscent of crocodilians than to Plesiosaurus. While many species have been assigned to Pliosaurus in the past, only six are firmly recognised as valid since a 2013 paper. Two additional species from Argentina were named in 2014 and 2018, but their assignment to the genus has not been fully confirmed in subsequent classifications.

The largest species, P. funkei and P. kevani, are among the largest pliosaurs ever discovered, with maximum sizes estimated at around 9–10 metres (30–33 ft) long. Even before both species were described in the scientific literature, their imposing size led some scientists to assign them various nicknames, the most famous being "Predator X", used to refer to the second known specimen of P. funkei. However, a large specimen composed entirely of cervical vertebrae discovered in Abingdon and putatively assigned to the genus could have reached a length exceeding 11 metres (36 ft). The elongated skull of Pliosaurus is robust, and those of the largest known species exceed 2 metres (6.6 ft) in length. The teeth are also robust and are trihedral in cross-section, which is one of the main diagnostic traits of the genus. The neck is one of the most recognizable features of the taxon, being relatively short compared to other plesiosaurs. The limbs of Pliosaurus were modified into flippers, with the anterior flippers of P. funkei reaching up to 3 metres (9.8 ft) in length.

Phylogenetic classifications place Pliosaurus in a relatively derived position within the Thalassophonea, with the genus often being recovered as related to Gallardosaurus. Like other plesiosaurs, Pliosaurus was well-adapted to aquatic life, using its flippers for a method of swimming known as subaqueous flight. Pliosaurid skulls are reinforced to better withstand the stresses of their feeding. The long, robust snouts of the various Pliosaurus species were used to capture large prey, piercing them with the numerous trihedral teeth at the back of the jaws. P. kevani would have achieved a bite force ranging from 9,600 to 48,000 newtons (2,160 to 10,790 lbf) depending on the area of the jaws, and its rostrum would likely have had an electro-sensitive organ to detect nearby prey. The fossil record shows that Pliosaurus shared its habitat with a variety of other animals, including invertebrates, fish, thalattosuchians, ichthyosaurs, and other plesiosaurs, which would undoubtedly have constituted prey for the larger species.

In 1824, William Daniel Conybeare established a new species of the genus Plesiosaurus, Plesiosaurus giganteus, to include all plesiosaurian specimens with shortened cervical vertebrae. Among the specimens that were classified in this taxon was a partial skeleton discovered in Market Rasen, Lincolnshire, England, collected by William Buckland. This specimen has since been stored in the Oxford University Museum of Natural History, where it has since been catalogued as OUMNH J.9245. Other fossils associated with this same individual are also stored in the museum, but their records tend to vary depending on the studies describing them since. Together it consists of teeth, a mandible, upper jaw, a partial spinal column, a femur, a tibia, and a fibula. Its ontogenetic stage is unknown, but the incomplete ossification of the proximal convexity of the tibia suggests that it is a juvenile or subadult specimen. In his article, Conybeare only refers specimens to this species, designating no type specimens and giving no detailed anatomical descriptions. As the referred specimens were then insufficiently characterized, the name Plesiosaurus giganteus was generally perceived as invalid in subsequent works, having since become a nomen oblitum.

In 1841, Richard Owen described the anatomy of the jaws of the Market Rasen specimen. Based on several distinctive features, he decided to make it the holotype of a subgenus accompanying a new species of Plesiosaurus, which he named Plesiosaurus (Pleiosaurus) brachydeirus. The name Pleiosaurus comes from the Ancient Greek πλειων (pleion, "more"), and σαῦρος (saûros, "lizard"), Owen named it this way because the appearance of the specimen described is closer to those of crocodilians than to those of other species then attributed to Plesiosaurus. The specific name brachydeirus also comes from Ancient Greek and comes from the words βραχύς (brakhús, "short"), and δειρή (deirḗ, "neck" or "throat"), in reference to its cervical vertebrae. Later that same year, Owen described the postcranial parts of the skeleton and relegated this taxon to a separate genus, but spelling it as Pliosaurus. In 1869, the same author erroneously claimed that another species formerly attributed to Pliosaurus, P. grandis, would be the type species of this genus. In 1871, John Phillips corrected most of Owen's taxonomic errors, recognizing P. brachydeirus as the type species and using the original spelling Pleiosaurus. Furthermore, the holotypic material of P. grandis is considered by many authors to be non-diagnostic and cannot be proven to belong to the genus. However, despite Phillips's requirement, the genus name Pliosaurus has since entered into universal usage and must be maintained according to the rule of article 33.3.1 of the ICZN. Currently, only the holotype and the fossils attributed to it constitute the only known specimen of this species.

In 1948, Nestor Novozhilov named the species P. rossicus on the basis of two more or less partial specimens discovered in two mines in the Lower Volga Basin in Russia (hence the name), and which have since been housed in the paleontological collections of the country's scientific academy. The holotype consists of a skull and postcranial remains from a relatively small specimen, since catalogued as PIN 304/1. This same specimen, which was originally a complete skeleton, was largely destroyed ten years earlier due to the exploitation of the oil shale from which it was discovered. However, pectoral elements associated with the latter are described by the same author in 1964. Based on its small size and poorly developed anatomical elements on the scapula, the holotype is interpreted as a juvenile. The second specimen, more imposing and since catalogued as PIN 2440/1, was discovered in May 1945 as a complete skeleton, but like the holotype, it was largely destroyed by mining operations. The only surviving remains of the latter are a rostrum, a proximal part of a humerus, a phalanx, and ribs fragments, which were originally described in 1947 as coming from a P. grandis by Anatoly Rozhdestvensky. In 1971, Beverly Halstead reclassified this species in the genus Liopleurodon because of its short mandibular symphysis (where the two halves of the lower jaw connect), and assigned the second specimen a complete hindlimb. However, it later turned out that this hindlimb actually originates from the holotype of the contemporary species P. irgisensis, which has since been recognised as a dubious. Although Halstead's classification was long recognised as valid, it was questioned in a 2001 thesis by Leslie F. Noè, who noted that, due to the shape of the teeth and the length of the mandibular symphysis, the species might represent a new genus. In a revision published in 2012, Espen M. Knutsen nevertheless reassigned the species to its original genus on the basis of diagnostic features shared with other lineages within the genus.

The species P. funkei was described in 2012 by Knutsen and colleagues based on two large specimens discovered in the Norwegian archipelago of Svalbard. The holotype, catalogued as PMO 214.135, consists of a partial skeleton preserving the anterior part of the jaws with teeth, various more or less preserved vertebrae, a complete right coracoid, a nearly complete right flipper, as well as ribs and gastralia (abdominal ribs). The larger referred specimen, catalogued as PMO 214.136, includes a partial skull preserving mostly its left posterior part, a few vertebrae and several unidentifiable fragmentary bones. Based on various morphological and histological characteristics, particularly in terms of increased bone density, these two specimens appear to have been adults. The fossils were collected at 2 km (1.2 mi) intervals during eight seasons of fieldwork conducted from 2004 to 2012 by Jørn Hurum in the southern Sassenfjorden. More precisely, they were discovered in 2006, excavated over the following two years, and officially reported at a Society of Vertebrate Paleontology conference in 2009, where their affinity with the genus Pliosaurus was already noted. Due to the Arctic climate of Svalbard, the specimens were subjected to repeated freeze-thaw cycles before collection, extensively fracturing and degrading the material. The specific name honours Bjørn Funke, the discoverer of the holotype, and his wife May-Liss Knudsen Funke, for their years of voluntary service to the paleontological collections of the University of Oslo Natural History Museum, where the specimens have since been housed. In their article, Knutsen and colleagues conclude that it cannot be definitively excluded that P. funkei and P. rossicus would represent different ontogenetic stages of the same species based on the available material. Nevertheless, the two taxa are still maintained as distinct on the basis that their respective holotypes exhibit proportionally very different humeral lengths. The rather remarkable size of the fossils led the Hurum's paleontological teams to nickname them "The Monster" for the holotype and "Predator X" for the referred specimen, which gave the taxon significant media coverage even before they were firmly described. This popularity led the species to appear in a 2009 documentary broadcast on the American television channel History, then in a 2010 low-budget science fiction horror film.

In a 2013 paper published in the mega journal PLOS ONE, Roger B. J. Benson and colleagues described three additional species of Pliosaurus, all of whose fossils were discovered in England. The first of these three species described was P. kevani, whose holotype consists of a large, nearly complete skull of a probable adult, which was discovered in a cliff at Osmington Mills Bay, Dorset. This specimen was collected over a period of eight years via fragmented parts weighing up to 60 kg (130 lb). Most of the fossils comprising the skull were taken without prior permission from loose or fallen boulders, while others were collected locally and purchased from landowners. The specimen was quickly identified as a pliosaurid by geologist Richard Edmonds. The remarkable size and completeness of this specimen, like "The Monster" and "Predator X", gave it widespread media coverage, to the point of earning the nickname "Weymouth Bay pliosaur" before its official description. The Dorset Museum's acquisition of the specimen was publicly announced in 2009, where it has since been catalogued as DORCM G.13,675. However, it was not officially opened by David Attenborough until July 2011. This discrepancy is due in part to the time taken to prepare the fossils, which took 200 hours for the lower jaw and over 365 hours for the rest of the skull. The specific name for this taxon honours Kevan Sheehan, owner of a small cafe overlooking the sea at Osmington Mills, who collected most of the holotype specimen during daily walks along the shore. In addition to the holotype skull, Benson and colleagues tentatively assigned two other large pliosaurid specimens discovered in the Cambridgeshire as P. cf. kevani os the basis of their tooth morphology. The first consists of a large, mostly postcranial skeleton catalogued as CAMSM J.35990, discovered in Stretham, while the second is a single tooth catalogued as LEICT G418.1965.108, discovered in Ely, which are stored at the Sedgwick Museum of Earth Sciences and the Leicester Museum & Art Gallery respectively.