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 southwestern 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 long.