Claosaurus (/ˌkleɪəˈsɔːrəs/ KLAY-ə-SOR-əs; Greek κλάω, klao meaning 'broken' and σαῦρος, sauros meaning 'lizard'; "broken lizard", referring to the odd position of the fossils when discovered) is a genus of hadrosauroid dinosaur that lived during the Late Cretaceous Period (Santonian-Campanian).

[2] 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 non-hadrosaurid relative of true hadrosaurids within the clade Hadrosauromorpha.

[3] Evidence of its existence was first found in the Niobrara Formation near the Smoky Hill River in Kansas, United States in the form of partial skull fragments and as an articulated postcranial skeleton.

This was corrected by Joseph Gregory in 1948, who found three toe bones from the right foot of a large hadrosaur in the Yale collections that had comparable preservation to the Pierre Shale turtle remains and were associated with labels in Wieland's handwriting.

[7] Reports of gastroliths, or stomach stones, in Claosaurus are actually based on a probable double misidentification.