Polacanthus (from the Ancient Greek polys-/πολύς- "many" and akantha/ἄκανθα "thorn" or "prickle") is an extinct genus of ankylosaurian dinosaurs from the early Cretaceous (130–125 million years ago) of England. Several species have been named in the genus Polacanthus, but only the type species, Polacanthus foxii, is currently seen as valid. There are not many fossil remains of this dinosaur, and some important anatomical features, such as its skull, are poorly known. It grew to about 4–5 metres (13–16 ft) long. Its body was covered with armour plates and spikes. It may be a basal member of the Nodosauridae or part of a separate family, the Polacanthidae.

Polacanthus was a medium-sized ankylosaur. In 2010, Gregory S. Paul estimated its length at 5 metres (16 ft) and its weight at 2 tonnes (2.2 short tons). Thomas Holtz gave a lower estimation of 4 metres (13 ft) and 227–454 kilograms (500–1,001 lb) in 2012. Its hindlimbs are relatively long for an ankylosaur, with the holotype right femur measuring about 555 millimetres (21.9 in) in length.

In 2011 Barrett e.a. indicated two possible unique traits, autapomorphies: the floor of the neural canal is deeply cut by a groove with a V-shaped transverse profile; the caudal spikes have triangular bases in side view and narrow points. In 2020, a study concluded to a single autapomorphy: the ischia at half length curve towards each other, their rear ends touching at their inner sides.

The subsequent describers have always dedicated much effort at restoring the armour configuration. Hulke understood that Polacanthus had a large "pelvic shield" or "sacral shield", a single fused sheet of dermal bone over its hips (sacral area) which perhaps was not attached to the underlying bone and decorated with tubercles. This feature is shared with other "polacanthine" (basal nodosaurids) dinosaurs such as Gastonia and Mymoorapelta. With the holotype, this shield is 108 centimetres wide and 90 centimetres long. It features four horizontal rows of larger keeled osteoderms per side, surrounded by smaller ossicles.

These latter are sometimes completely fused to form flat armour plates. Hulke thought that on the tail there were two rows of keeled osteoderms per side. Of a set of spikes found with the fossil, he assumed they had adorned the sides of the rump. A different arrangement was hypothesised by Nopcsa. He thought that both the tail and the front of the body including the neck featured two parallel rows of spikes, one per side. On the front body each row would have consisted of five spikes and he claimed that seven of these had been conserved with the fossil, five of the right side and two of the left. The tail rows would have consisted of twenty-two shorter pairs, fifteen spikes being still extant, eight of the left side and seven of the right. As the spikes are asymmetrical their position can more or less be deduced. Blows in 1987 basically agreed with Nopcsa but also distinguished three spike types, a Type A, B and C, allowing him to classify additional fossil finds, which often differed from the holotype spikes in several details. In 2013 a footprint was found by Henley Hobbs and his father on the Isle of Wight. Now a picture of the footprint is inside the dinosaur farm.

Polacanthus foxii was discovered by the Reverend William Fox on the Isle of Wight in early 1865, at Barnes High at the southwest coast. Fox at first planned to have his friend Alfred Tennyson name the new dinosaur during a meeting on 23 July 1865, when the remains were shown to paleontologist Richard Owen. Tennyson proposed Euacanthus Vectianus but this name was ultimately rejected. In September 1865, Fox in a lecture to the British Association reported on the find and let it be named Polacanthus foxii by Owen, hereby perhaps circumventing the convention that an author does not name a taxon after himself. The text of the lecture, only published in 1866, was more or less reproduced by him in anonymous articles in the Geological Magazine and the Illustrated London News of 16 September 1865. This procedure caused some confusion as no corresponding 1865 publication by Owen exists. Some have therefore contended that Thomas Huxley in 1867 became the author of the name, others give Fox, Owen or "Anonymous" as the author. The generic name is derived from Greek πολύς, polys, "many" and ἄκανθα, akantha, "thorn", in reference to the many spikes of the armour. The specific name honours Fox.

The holotype, NHMUK PV R175, was found in a layer of the Upper Wessex Formation dating from the Barremian. It is an incomplete skeleton with the head, neck, anterior armour and forelimbs missing but including dorsal vertebrae, a sacral rod of five dorsosacrals, the sacrum, most of the pelvis, most of the left hindleg, the right thighbone, twenty-two tail vertebrae, ribs, chevrons, ossified tendons, a pelvic shield, twenty-two spikes and numerous ossicles. The skeleton was in 1881 studied by John Whitaker Hulke, while it was still in the possession of Fox. Hulke published the first detailed description of the find, noting that the specimen had badly deteriorated over the years, the dermal armour having almost fully fallen apart. The same year Fox died, his collection was acquired by the British Museum of Natural History, including the Polacanthus fossil. This was after arrival in the museum in 1882, reassembled by preparator Caleb Barlow, painstakingly putting all the pieces together with Canada balsam, much to the wonder of Hulke who in 1881 had called this a hopeless undertaking. This allowed Hulke to redescribe the specimen in 1887, with a special attention to the armour arrangement. In 1905, when it was mounted by the museum, the specimen was again described by Franz Nopcsa who for the first time provided an illustration of the possible spike configuration. Later, the specimen was stored in the museum basement.

Numerous other specimens from Wight and Great Britain have since been referred to Polacanthus. These mostly consist of single bones or armour elements. Several specimens that were discovered prior to the holotype were at various points considered to belong to Polacanthus. In 1843 John Edward Lee reported the discovery on Wight of three such specimens, consisting only of armour pieces. They were already lost before the description was published. In 1859, geologist Ernest P. Wilkins mentioned the presence in his collection of numerous scutes, spikes and vertebrae from Wight, referred by him to Hylaeosaurus. After his death, his collection was moved several times and the pieces were lost.