Scelidosaurus harrisonii (meaning “limb lizard,” from the Greek skelos for leg and sauros for lizard or reptile) is an early armored ornithischian dinosaur from the Early Jurassic period, living approximately 193 million years ago in what is now Dorset, England. Its fossils have also been found in Ireland, making it the only classified dinosaur from that country. The species is named after James Harrison, the fossil collector who discovered the original specimen in 1858 at Charmouth on the Dorset coast. Despite being known since 1859 and representing the first ever more or less complete dinosaur skeleton discovered anywhere in the world, Scelidosaurus was never properly described until University of Cambridge paleontologist David Norman completed a comprehensive redescription published in four papers across 2020, over 160 years after the fossil was found.

Scelidosaurus was a medium-sized herbivore estimated at 4 to 5 metres (13 to 16 feet) in length and approximately 250 kilograms in body mass. It was covered in osteoderms running along its back, sides, and tail, providing substantial protection against predators. Its relationship to the later armored dinosaur groups remains debated: recent analysis suggests it may be an ancestor of the ankylosaurs alone rather than a common ancestor of both ankylosaurs and stegosaurs as previously thought. Scelidosaurus is the most completely known dinosaur from the British Isles and the only classified dinosaur from Ireland.