Calamosaurus, signifying "reed lizard," existed as a genus of diminutive theropod dinosaur within the Lower Cretaceous Wessex Formation of the Isle of Wight, England, during the Barremian age.

Due to the limited fossil evidence available, Calamosaurus has garnered limited attention. Frequently, it has been mixed with Calamospondylus as a component of a taxonomic puzzle, even though there are no equivalent remains from both genera. Recent evaluations have categorized it as an uncertain theropod, although it could potentially belong to the Coelurosauria group.

As a potential primitive coelurosaur, Calamosaurus likely represented a nimble, two-legged carnivore. According to scientists, the creature's size when alive would have ranged from approximately 3 to 5 meters (9.8 to 16.4 feet), with a relatively small head in proportion to the structure of its neck vertebrae.