Scanisaurus is a dubious genus of plesiosaur that lived in what is now Sweden and Russia during the Campanian stage of the Late Cretaceous period. The name Scanisaurus means "Skåne lizard", Skåne being the southernmost province of Sweden, where a majority of the fossils referred to the genus have been recovered. The genus contains one species, S. nazarowi, described in 1911 by Nikolay Bogolyubov as a species of Cimoliasaurus based on a single vertebral centrum discovered near Orenburg, Russia. the species was moved into its own genus by Per-Ove Persson in 1959 after several differences were observed between the Russian centra and new fossils from Skåne and the type species of Cimoliasaurus. Due to the limited type material and the lack of diagnostic features in the Swedish fossils confidently separating Scanisaurus from other Late Cretaceous elasmosaurids, the genus is of questionable validity, though it continues to be used in practice.

Scanisaurus fossils have mainly been found in the Kristianstad Basin in northeastern Skåne, where they represent the most common plesiosaur fossils. Scanisaurus shared its environment with a diverse marine fauna, including many other marine reptiles. It would have been a middle trophic-level predator, about 4–5 meters (13–16 ft) in length, and would have been able to feed both in open water and on the sea floor, likely feeding mainly on small prey such as small fish or belemnites.

In 1911, Russian paleontologist Nikolay Bogolyubov described a Late Cretaceous posterior cervical (neck) vertebral centrum discovered near Orenburg, Russia. Bogolyubov referred the centrum to the plesiosaur genus Cimoliasaurus and believed it to represent a new species, which he named C. nazarowi. Bogolyubov compared the centrum with those of other plesiosaurs and found it to be most similar to a vertebra referred to Cimoliasaurus sp. from the Cenomanian Quiriquina Formation of Quiriquina Island, Chile and to vertebrae referred to Cimoliasaurus magnus, the type species of Cimoliasaurus. The main distinguishing feature used by Bogolyubov to justify the creation of a new species was that his centrum was wider than other centra referred to Cimoliasaurus.

In 1959, Swedish paleontologist Per-Ove Persson examined the fragmentary plesiosaur fossil record of Late Cretaceous Skåne in southern Sweden, primarily recovered from fossil sites within the Kristianstad Basin (many from the island of Ivö) and from south-western Skåne. He found that cervical vertebral centra from Skåne accorded so well with the vertebra described by Bogolyubov that they "must belong to one and the same genus". Furthermore, Persson noted several differences between Bogolyubov's centrum and the Swedish material and the fossils of C. magnus, and considered C. nazarowi distinct enough to warrant being placed in a separate genus. Persson named this new genus Scanisaurus, meaning "Skåne lizard". Persson noted that S. nazarowi remained an "undefinable" species since it remains based on only a single vertebral centrum, but felt confident that the Swedish material was referrable to the species since it did not differ in any essential points from Bogolyubov's fossil. Persson noted that the Swedish fossils were the same species "with a fairly great degree of probability" and provisionally designated them as S. cf. nazarowi.

There were three principal characteristics Persson perceived to differentiate Scanisaurus from Cimoliasaurus. First, in Cimoliasaurus, the ribs were fused to the vertebrae with at least the pre-pectoral centra, whilst in Scanisaurus the cervical ribs were fused to the centra by only the sutures. Second, the length of the posterior cervical centra decreased towards the head in Cimoliasaurus, while the opposite was true in Scanisaurus. Third, the cervical centra of Scanisaurus were broader proportional to their length than the corresponding centra of Cimoliasaurus.

Because centra referred to S. cf. nazarowi were far more common in the Swedish fossil sites compared to centra from other plesiosaurs, Persson concluded that S. cf. nazarowi was "obviously the most common plesiosaurian" in Late Cretaceous Skåne. With this in mind, he also referred the most common type of plesiosaur teeth found, some of which had been found in association with S. cf. nazarowi vertebrae, to the species as well, alongside associated ossifications of humeri and femora.

In 1995, in an examination of material referred to the invalid species Plesiosaurus houzeaui (found in Belgium), French paleontologist Nathalie Bardet and Belgian paleontologist Pascal Godefroit discussed other questionable plesiosaur species from Europe. Bardet and Godefroit noted that though Persson had referred the Swedish material to several different elasmosaurid genera, including Scanisaurus and Elasmosaurus, the fossils only possessed the necessary characteristics to be referred to the Elasmosauridae, not a particular genus or species. Though the material referred to S. cf. nazarowi, consisting of vertebrae, teeth and limb bones, was more complete than the material referred to Elasmosaurus, it was deemed to lack any diagnostic features with which it could be differentiated from other Late Cretaceous elasmosaurids. Though Scanisaurus for this reason is typically no longer considered a valid taxon (constituting a nomen dubium), the name continues to be used in practice.

In 1996, Persson provisionally referred a crushed reptile skull recovered from Ignaberga quarry in the Kristianstad Basin to Scanisaurus sp., since two tooth fragments associated with the fossil showed the same striation pattern as in the teeth referred to S. cf. nazrowi. Though the skull is too crushed to give much useful anatomical information, it is the only cranial fossil referred to Scanisaurus (with the exception of teeth) and demonstrates that its head was comparatively larger than the heads of other dolichodiran plesiosaurs.