Temnodontosaurus (meaning "cutting-tooth lizard") is an extinct genus of large ichthyosaurs that lived during the Lower Jurassic in what is now Europe and possibly Chile. The first known fossil is a specimen consisting of a complete skull and partial skeleton discovered on a cliff by Joseph and Mary Anning around the early 1810s in Dorset, England. The anatomy of this specimen was subsequently analyzed in a series of articles written by Sir Everard Home between 1814 and 1819, making it the very first ichthyosaur to have been scientifically described. In 1822, the specimen was assigned to the genus Ichthyosaurus by William Conybeare, and more precisely to the species I. platyodon. Noting the large dental differences with other species of Ichthyosaurus, Richard Lydekker suggested in 1889 moving this species into a separate genus, which he named Temnodontosaurus. While many species have been assigned to the genus, only five are currently recognized as valid, the others being considered as synonymous, doubtful or possibly belonging to other taxa.

Generally estimated at 9 m (30 ft) long, Temnodontosaurus is one of the largest known ichthyosaurs, although not as imposing as some Triassic forms. Specimens assigned to the genus may nevertheless have reached larger measurements. As an ichthyosaur, Temnodontosaurus had flippers for limbs and a fin on the tail. Boasting eye sockets measuring more than 25 cm (9.8 in) wide, Temnodontosaurus quite possibly had the largest eyes known in the entire animal kingdom, rivaling in size those of the colossal squid. The snout appears to be longer than the mandible, being equipped with several sharp teeth (hence its name). On the basis of numerous very complete skeletons, it is estimated that the animal had at least more than 40 presacral vertebrae. Temnodontosaurus is a basal representative of the parvipelvian subgroup of ichthyosaurs, in addition to being its largest representative. A monotypic family, Temnodontosauridae, was even established in 1974 to include the genus. Various phylogenetic analyses as well as diagnostic problems concerning the genus make it, for the moment, a polyphyletic taxon (unnatural grouping), and therefore in need of revision.

Temnodontosaurus is historically the very first ichthyosaur to have been scientifically described. Around 1810, a certain Joseph Anning discovered the first skull of the taxon on the cliffs of Black Ven, between the town of Lyme Regis and the village of Charmouth, two localities located in the county of Dorset, in the south of England. The remaining skeleton was later discovered by his sister, the now famous Mary Anning, in 1812. Although other ichthyosaur skeletons had been discovered locally and elsewhere, this particular specimen was the first to attract attention of the scientific community. After the discovery was announced in the press, the specimen was purchased by the lord of a local manor, Henry Hoste Henley, for a price of £23. Subsequently, Henley passed the fossils on to the naturalist William Bullock, who put them on display in the collections of his museum in London. In 1819, Bullock's own collection was sold to the Natural History Museum in London for a price of around £47. The specimen, now cataloged as NHMUK PV R 1158, is still currently housed at this museum, although the postcranial remains have since been lost.

Beginning in 1814, Sir Everard Home wrote a series of six papers for the Royal Society describing the specimen, initially identifying it as a crocodile. Perplexed as to the real nature of the fossil, Home kept changing his mind about its classification, thinking that it would be a fish, then as an animal sharing affinities with the platypus, which was then recently described at that time. Finally, in 1819, he thought that the fossil represented an animal that embodied an intermediate form between salamanders and lizards, which led him to erect the genus name Proteosaurus (originally written as Proteo-Saurus). In 1821, Henry De la Beche and his colleague William Daniel Conybeare made the very first scientific description of Ichthyosaurus, but did not name any species. Although being initially a nomen nudum, this generic name was already proposed in 1818 by Charles Konig, but was thus chosen as the definitive scientific name of this genus, Proteosaurus having since become a nomen oblitum. In their article, De la Beche and Conybeare refer several additional fossils discovered at Black Ven to this genus, also including the specimen originally described by Home, and finally identify it as a marine reptile. It was in 1822 that De la Beche named three species of Ichthyosaurus on the basis of several anatomical differences distinguishing the specimens, one of them being I. platyodon. He nevertheless announces that future descriptions will be done with the help of Conybeare. However, it was Conybeare himself who described the fossils the same year, attributing the largest specimens to I. platyodon, The specific name platyodon comes from the Ancient Greek πλατύς (platús, "flat", "broad"), and ὀδούς (odoús, "tooth"), all meaning "flat teeth", in reference to the rather distinctive dentition of this species.

In 1889, Henry Alleyne Nicholson and Richard Lydekker published a two-volume work that served as an introduction to the rules of paleontology for students. However, it is in the second volume that the two paleontologists gave a very detailed description of numerous prehistoric vertebrates, and during which the taxonomy of I. platyodon took another direction. Indeed, in his correction notes, Lydekker noticed that the teeth of I. platyodon had great differences from those of other previously recognized species of Ichthyosaurus and suggest that the latter could be the type species of a completely new genus of ichthyosaurs, which he named Temnodontosaurus. This generic name is formed from the Ancient Greek τέμνω (temnō, "to cut"), ὀδούς (odoús, "tooth"), and σαῦρος (saûros, "lizard"), to give "cutting-tooth lizard". In a broad review of fossil vertebrates published in 1902, Oliver Perry Hay suggested that because the name Proteosaurus technically took precedence over Ichthyosaurus, he then displaced I. platyodon as the type species of that genus, then renamed Proteosaurus platyodon. In 1972, Christopher McGowan again used this combination proposed by Hay (although not mentioned), but the latter revised his judgment two years later, in 1974, in which he moved this species to Temnodontosaurus, as originally proposed by Lydekker. The holotype of Temnodontosaurus platyodon consisted of a single tooth which was preserved by the Geological Society of London. As the latter has since been noted as lost in 1960, McGowan designated specimen NHMUK PV R 2003 as the neotype of this taxon. This specimen, already mentioned as a representative of the species by Richard Owen in 1881, was originally discovered and partly collected by Mary Anning in July 1832 in Lyme Regis. After the discovery, she sold the find to Thomas Hawkins, who himself sold the specimen to the Natural History Museum in London in 1834 for a price of £210.

In 1843, Carl von Theodori [de] described a new species of Ichthyosaurus, I. trigonodon, which he described as "colossal", based on an imposing specimen comprising a complete skull and a partial postcranial skeleton discovered in the town of Holzmaden in the state of Baden-Württemberg, Germany. The specific name comes from Ancient Greek τρίγωνον (trígônon, "triangle") and ὀδούς (odoús, "tooth"), in reference to the dental crown which is visibly triangular in this species. In 1854, von Theodori made a much more in-depth description of the holotype specimen. In 1889, only some time before he established the genus Temnodontosaurus, Lydekker noted that the dentition of I. trigonodon was quite similar to that of I. platyodon. Based on these dental characteristics, he moved this species to the genus Temnodontosaurus the following year, consequently being renamed T. trigonodon. In 1931, Friedrich von Huene transferred this species to the genus Leptopterygius. In 1998, Michael W. Maisch moved the species again to the genus Temnodontosaurus, and attributed to this taxon other, mostly very complete, specimens having been discovered in Germany and France. This classification has since been retained in subsequent works, to the point that a large specimen discovered in England in 2021, nicknamed as the 'Rutland Sea Dragon', was even considered as the first probable representative within this country. However, a 2023 morphological study conducted by Rebecca F. Bennion and colleagues shows that the holotype specimen differs in cranial and dental traits from other specimens since assigned to the species. The authors therefore suggest that a future re-evaluation is necessary for better a diagnosis of T. trigonodon.

In 1857, an almost complete skeleton of a large ichthyosaur was discovered north of Whitby, Yorkshire. The latter was also found near another skeleton, that of a pliosaur, which is today recognized as the holotype specimen of Rhomaleosaurus cramptoni. Shortly after its discovery, the ichthyosaurian skeleton was subsequently sent to the Yorkshire Museum, where it was cataloged as YORYM 497. The following year, 1858, Owen examined the specimen and classified it as distinct from I. platyodon, attributing it to a completely new species which he named I. crassimanus. However, the latter was never scientifically described by Owen, although it is briefly mentioned in a work by John Phillips and Robert Etheridge published in 1875. It was in 1876 that John Frederick Blake made the first scientific description of the animal, although he did so only very briefly. In 1889, Lydekker considered this species as a potential junior synonym of I. trigonodon, an opinion which was followed by numerous authors until around the beginning of the 20th century. In 1930, Sidney Melmore made the first in-depth description of I. crassimanus based on the holotype specimen, restoring the distinct status of the species. In his revision published in 1974, McGowan synonymized I. crassimanus with the proposed taxon Stenopterygius acutirostis, also attributing other specimens discovered in the original locality. In 2003, McGowan and Ryosuke Motani suggested that all specimens historically attributed to I. crassimanus appeared sufficiently different from T. platyodon and T. trigonodon to belong to a distinct species of the genus Temnodontosaurus, being renamed T. crassimanus. However, they note that further research could question its validity. Long remaining an under-analyzed taxon, it was in 2020 that Emily J. Swaby wrote a thesis considerably revising it. Contrary to the suggestion previously made by McGowan and Motani, Swaby maintains the attribution of this species to Temnodontosaurus. The descriptions of this same thesis were finally published the following year in a study co-authored with Daniel R. Lomax.

In 1880, Harry Govier Seeley described the species I. zetlandicus on the basis of a well-preserved skull loaned by an Earl of Shetland (hence its name) around an unspecified date to the Sedgwick Museum of Earth Sciences in Cambridge. This skull, cataloged as CAMSM J35176, was discovered in the coasts of Whitby, near the locality where T. crassimanus was already discovered. In 1922, von Huene moved the species within Stenopterygius. In 1974, McGowan considered S. zetlandicus as a synonym of S. acutirostris, before this species was itself synonymized with T. acutirostris from 1997. In 2022, Antoine Laboury and colleagues reestablished the validity of the species by redescribing CAMSM J35176, but moved it to the genus Temnodontosaurus, being renamed T. zetlandicus. In their description, they attribute another specimen to the taxon, cataloged as MNHNL TU885, a partial skull which was originally discovered in Schouweiler, southern Luxembourg.