Hydrorion (meaning 'water hunter') is a genus of plesiosaur from the Toarcian Age of the Lower Jurassic. It is only known from multiple specimens, all discovered in the Posidonia Shale of south-western Germany. The only species of Hydrorion is H. brachypterygius, which was originally described as a species of Plesiosaurus and then Microcleidus. It was a relatively small plesiosaur, with the largest specimen measuring 4 m (13 ft) long.