| 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 |
| Diet: | Herbivore |
| 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.