Pterosaurs (/ˈtɛrɵsɔr/; meaning "winged lizard") were flying Reptiles that make up the clade or order Pterosauria. They existed from the Late Triassic to the end of the Cretaceous period (228-66 million BC). Pterosaurs are the earliest Vertebrates known to have evolved powered flight. Their wings were formed by a membrane of skin, muscle, and other tissues stretching from the ankles to a dramatically lengthened fourth finger. Early species had long, fully toothed jaws and long tails, while later forms had a highly reduced tail, and some lacked teeth. Many sported furry coats made up of hair-like filaments known as pycnofibers, which covered their bodies and parts of their wings. Pterosaurs spanned a wide range of adult sizes, from the very small Nemicolopterus to the largest known flying Creatures of all time, including Quetzalcoatlus and Hatzegopteryx.

Pterosaurs are often referred to in the popular media and by the general public as flying Dinosaurs, but this is scientifically incorrect. The term "Dinosaur" is restricted to just those animals descended from the last common Reptilian ancestor of the groups Saurischia and Ornithischia (clade Dinosauria, which includes Birds), and current scientific consensus is that this group excludes the Pterosaurs, as well as the various groups of extinct marine Reptiles, such as Ichthyosaurs, Plesiosaurs, and Mosasaurs. Like the Dinosaurs, and unlike these other Reptiles, Pterosaurs are closer to Birds than to Crocodilians or any other living Reptile. Pterosaurs are also colloquially referred to as Pterodactyls, particularly by journalists. Technically, "Pterodactyl" refers only to members of the genus Pterodactylus, and more broadly to members of the suborder Pterodactyloidea of the Pterosaurs.

They died out from the Cretaceous-Paleogene Extinction Event.