|Named By:||Othniel Charles Marsh in 1890|
|Time Period:||Late Cretaceous, 87-82 Ma|
|Location:||USA, Kansas - Niobrara Formation, Montana - Pierre Shale Formation, South Dakota - Pierre Shale Formation, Texas - Aguja Formation|
|Size:||Roughly about 3.5 meters long|
|Fossil(s):||Partial remains of a few individuals|
|Classification:||| Chordata | Reptilia | Dinosauria | Ornithischia | Ornithopoda | Hadrosauroidea ||
|Also known as:||| Hadrosaurus agilis ||
Claosaurus ( KLAY-o-SAWR-as; Greek klao, klao meaning 'broken' and sauros, sauros meaning 'lizard'; "broken lizard", referring to the odd position of the fossils when discovered) is a genus of primitive hadrosaurian (early duck-billed dinosaur) that lived during the Late Cretaceous Period (Campanian).
Traditionally classified as an early member of the family Hadrosauridae, a 2008 analysis found Claosaurus agilis to be outside of the clade containing Hadrosaurus and other hadrosaurids, making it the closest not-hadrosaurid relative of true hadrosaurids within the clade Hadrosauria.