Simosaurus is an extinct genus of marine reptile within the superorder Sauropterygia from the Middle Triassic of central Europe.

[2] The type species of Simosaurus, S. gaillardoti, was named by German paleontologist Christian Erich Hermann von Meyer in 1842.

Material found in France includes the holotype skull of S. gaillardoti and a partial mandible referred to S. mougeoti.

Although initially attributed to Simosaurus, the mandible was labeled as "Nothosaurus mougeoti" in one of von Meyer's later papers.

Von Meyer named S. guilelmi on the basis of this skull, noting that it was smaller and narrower than those of the type species.

The humerus has well-developed crests and the underside of the pectoral girdle is large, suggesting that the forelimbs had a powerful downstroke and provided most of the thrust required for swimming.

Simosaurus was probably a moderately powerful swimmer with a locomotion that was transitional between the lateral undulation of early sauropterygians and the strong flipper-driven swimming of plesiosaurs.

[2] Because it has blunt teeth, Simosaurus is often thought to have been durophagous, meaning that it ate organisms with hard shells.