Ornithocheirus (from Greek "ορνις", meaning bird, and "χειρ", meaning hand) was a huge pterosaur from the early Cretaceous period of Europe and South America. Along with Quetzalcoatlus, several large bone fragments from the Santana Formation of Brazil indicate that Ornithocheirus may have been one of the largest pterosaurs that ever lived, with a wingspan reaching almost 12 m (40 ft). If these poorly preserved remains do indeed belong to Ornithocheirus, it would have been approximately 3.50 m (11.5 ft) long (1.50 m (5 ft) of which belonged to the head) and stood approximately 3 m (10 ft) tall when on the ground on all fours. Despite this, the creature would only have weighed as much as a grown man (70 kg), thanks to hollow bones filled with air sacks. Ornithocheirus was the earliest giant pterosaur, living 125 million years ago. Most other large-sized genera appeared around 90 million years ago.

Though many species have been named in the genus Ornithocheirus over the years, only three (O. simus, O. mesembrinus, and a yet-unnamed third species) are currently recognized as valid by most pterosaur researchers. Part of the confusion lies in the fact that the type specimen of Ornithocheirus is poorly preserved, and difficult to tell apart from related pterosaurs (such as Anhanguera). The famous "keel-jawed" pterosaur specimens originally assigned to Ornithocheirus (as O. mesembrinus) have even been assigned to their own genus (Tropeognathus mesembrinus), though today many researchers, including David Unwin, agree that these are indeed species of Ornithocheirus (making Tropeognathus a junior synonym).