Polacanthus, deriving its name from the Ancient Greek polys- "many" and akantha "thorn" or "prickle", is an early armoured, spiked, plant-eating ankylosaurian dinosaur from the early Cretaceous period of England.
In the genus Polacanthus several species have been named but only the type species Polacanthus foxii is today seen as valid.
Polacanthus was a quadrupedal ornithischian or "bird-hipped" dinosaur. It lived 130 to 125 million years ago in what is now western Europe. Polacanthus foxii was named after a find on the Isle of Wight in 1865. There are not many fossil remains of this creature, and some important anatomical features, such as its skull, are poorly known. Early depictions often gave it a very generic head as it was only known from the rear half of the creature. It grew to about 5 metres (16 ft) long. Its body was covered with armour plates and spikes. It possibly was a basal member of the Nodosauridae.