Stenopterygius is an extinct genus of thunnosaur ichthyosaur known from Europe (England, France, Germany, Luxembourg and Switzerland).

One well-preserved fossil of Stenopterygius preserves traces of skin, from which the animal's coloration was discovered to be countershaded (darker on the back than the underbelly).

The type specimens of S. longifrons, S. megacephalus, and S. megalorhinus were all referred to this species, as the name I. triscissus has a priority over them.

Some specimens previously referred to S. megalorhinus, as well as the holotype of S. cuneiceps, were found to belong to a species of their own for which the binomen Stenopterygius uniter can be used.

Maisch also found that S. promegacephalus is a nomen dubium, as it is based on a juvenile specimen, and that the lectotype of S. hauffianus can be determined as Stenopterygius cf.

[2] Stenopterygius is known from the lectotype GPIT 43/0219-1 (also registered as GPIT-PV-30028 and PV 7532), a complete, articulated skeleton which preserved a very large embryo.

It was collected from the Harpoceras elegantulum-exaratum ammonoid subzones (more specifically Lias ε II3-4), Harpoceras falcifer zone, of the famous Posidonien-Schiefer lagerstätte (Posidonia Shale) of Holzmaden, dating to the early Toarcian stage of the Early Jurassic, about 182 million years ago.

Maisch referred to the type species 30 additional specimens, all came from Dobbertin of Mecklenburg-Vorpommern and Holzmaden, Germany and Dudelange, Luxembourg.

Maisch referred to this species 13 additional specimens, all came from various localities in England, France, Germany, Luxembourg and Switzerland.

[2] Additional materials were described by Hannah Caine and Michael J. Benton in 2011, from the early Toarcian Beacon Limestone of Strawberry Bank, Ilminster of England.

Fischer et al. (2011) performed a cladistic analysis that found Chacaicosaurus to be a basal thunnosaur which is placed outside both Stenopterygiidae and Ophthalmosauridae.

[17][18] Stenopterygius was a very fast swimmer, with a cruising speed similar to that of tuna, which is among the fastest of all living fishes.

[citation needed] In 2018, a Stenopterygius specimen was reported with evidence of having had blubber, which indicates that other ichthyosaurs and it were homeothermic ("warm blooded").