Gresslyosaurus (meaning "Amanz Gressly's lizard") is a genus of plateosaurian sauropodomorph dinosaur that lived during the Late Triassic period, around 214 to 204 million years ago, in France, Germany, Norway, Greenland and Switzerland.

The holotype of G. ingens, NMB BM 1, 10, 24, 53, 530-1, 1521, 1572-74, 1576-78, 1582, 1584-85, 1591, consists of postcranial remains discovered in the Late Triassic (late Norian-Rhaetian) Trossingen Formation or Knollenmergel Formation of northern Switzerland around 1840 by Amanz Gressly,[2] with more of the holotype being found between 1915 and 1942 by an unknown collector.

[6] The holotype of G. robustus, UT (GPIT) B, a set of postcrania, was collected from the Trossingen Formation of Germany by Quenstedt in 1879.

[6] The holotype of G. torgeri, HMN MB III, a set of postcrania, was collected from a Plateosaurus bonebed within the Trossingen Formation of Germany by Jaeckel in 1909 or 1910.

[3][14] In his 1865 book Die Urwelt der Schweiz, Oswald Heer notes that Amanz Gressly, who had been admitted to an asylum in the final years of his life, had become tormented by hallucinations that he had transformed into the Gresslyosaurus, his namesake dinosaur.