Parasaurolophus, whose name means "close to the crested lizard", is a genus of hadrosaurid ornithopod dinosaur that lived between 76.9 and 70.5 million years ago, during the Upper Cretaceous, the Campanian in what is now North America. It was a large herbivore that could reach more than 9 meters in length and weigh more than 5 tons, and could move as a biped and quadruped.

Currently, 3 species of Parasaurolophus are known: Parasaurolophus walkeri, Parasaurolophus tubicen, and Parasaurolophus cyrtocristatus. All of them can be distinguished from each other and have many differences. The first species named was Parasaurolophus walkeri. refers to a particular specimen from the Dinosaur Park Formation, but it has a simpler internal structure than Parasaurolophus tubicen, along with a straighter crest and a different internal structure than Parasaurolophus cyrtocristatus.

The second species was Parasaurolophus tubicen, which is the largest of the Parasaurolophus species. It lived in New Mexico, where three specimens are known, and can be differentiated from its other species. It had a long, straight crest, with a very complex interior compared to the other species. All known specimens of Parasaurolophus tubicen come from the De-Na-Zin Member of the Kirtland Formation.

Finally in 1961, John Ostrom named the third species, Parasaurolophus cyrtocristatus. Its three known specimens have been found in the Fruitland and Kaiparowits Formations of Utah and New Mexico. Of the Parasaurolophus species, Parasaurolophus cyrtocristatus is the smallest and has the most curved crest. Possessing both of the above characteristics, it was often believed to be female.

Parasaurolophus was a hadrosaurid, part of a diverse family of large Late Cretaceous ornithopods that are known for their variety of strange head ornaments, which were likely used for communication and increased hearing. This genus is known for its large and elaborate cranial crest, which forms a long, curved tube that projects upward and backward from the skull at its largest form.