Polacanthus was a nodosaur, a type of 'bird-hipped' dinosaur. It sported a thick coat of bony armour plates that covered its rump. Evenly spaced rows of fierce spikes ran along its sides and shoulders.

It was closely related to both the stegosaurs and ankylosaurs. All of these relied on armour or spines rather than fleetness of foot as defence against predators.

Fossil remains show that it was a squat, four-legged herbivore that ate ground vegetation. It is only known about from three fragmentary skeletons and many armoured plates. These have been found in southern England, particularly on the Isle of Wight.