Hurst Castle is an artillery fort established by Henry VIII on the Hurst Spit in Hampshire, England, between 1541 and 1544. It formed part of the king's Device Forts coastal protection programme against invasion from France and the Holy Roman Empire, and defended the western entrance to the Solent waterway. The early castle had a central keep and three bastions, and in 1547 was equipped with 26 guns. It was expensive to operate due to its size, but it formed one of the most powerful forts along the coast. During the English Civil War of the 1640s, Hurst was held by Parliament and was used briefly to detain King Charles I before his execution in 1649. It continued in use during the 18th century but fell into disrepair, the spit being frequented by smugglers.

Repairs were made during the French Revolutionary Wars and the Napoleonic Wars with France, and the castle was modernised to enable it to hold 24-pounder (10.8 kg) guns. Fresh fears of invasion followed in the 1850s, leading to heavier, 32-pounder (14.5 kg) armament being installed and new gun batteries being laid out on both sides of the castle. Technological developments rapidly made these defences obsolete, however, and a fresh phase of work between 1861 and 1874 created sixty-one gun positions in two long, granite-faced batteries alongside the older castle. These held very heavy weapons, including massive 12.5 inch, 38 ton (317 mm, 39,000 kg) rifled muzzle-loading guns. As the century progressed these too became outdated, and lighter, quick-firing guns were installed at the castle to replace them.

The castle formed part of a network of defences around the entrance to the Solent during the First World War, and was re-armed again during the Second World War. The military decommissioned the fort in 1956 and it passed into the control of the Ministry of Works. In the 21st century, it is run jointly by English Heritage and the Friends of Hurst Castle as a tourist attraction, receiving around 40,000 visitors during 2015. Coastal erosion has become a growing problem despite government intervention in protecting the spit. Four lighthouses have been built at Hurst from the 18th century onwards, one of which, a high lighthouse first opened in 1867, remains in active service.

In January 2021, local media reported that the castle was in urgent need of repairs due to coastal erosion, and the wall of the eastern wing partially collapsed on 26 February 2021 as the sea cut into and compromised its foundations. The World Monuments Fund included the castle on their 2022 World Monuments Watch list of most endangered sites.

In September 2022, it was one of six English castles included in a fundraising campaign by English Heritage to mitigate risks of destruction due to worsening coastal erosion. They hope to gather enough funds to repair and strengthen the sea walls around the castle.

Hurst Castle was built as a consequence of international tensions between England, France and the Holy Roman Empire in the final years of the reign of King Henry VIII. Traditionally the Crown had left coastal defences to local lords and communities, only taking a modest role in building and maintaining fortifications, and while France and the Empire remained in conflict, maritime raids were common but an actual invasion of England seemed unlikely. Modest defences based around simple blockhouses and towers existed in the south-west and along the Sussex coast, with a few more impressive works in the north of England, but in general the fortifications were limited in scale. Worsley's Tower, for example, built opposite the future site of Hurst Castle in the 1520s, was too small to hold powerful artillery and considered by surveyors in 1539 to be "one of the worst devised things" they had seen.

In 1533, Henry broke with Pope Clement VII over the annulment of his long-standing marriage to Catherine of Aragon. Catherine was the aunt of Charles V, the Holy Roman Emperor, who took the annulment as a personal insult. This resulted in France and the Empire declaring an alliance against Henry in 1538, and the Pope encouraging the two countries to attack England. An invasion of England appeared certain. In response, Henry issued an order, called a "device", in 1539, giving instructions for the "defence of the realm in time of invasion" and the construction of forts along the English coastline.

Hurst Castle was designed to protect the western entrance to the Solent, a body of water that led from the English Channel to the naval base at Portsmouth and, through Southampton Water, to the important port of Southampton. The castle was one of four fortifications that William Fitzwilliam, the Lord High Admiral, and William Paulet recommended building to strengthen the defences along the Solent; the others were at East and West Cowes, and Calshot. It was positioned on the Hurst Spit, a strip of shingle sheltering saltmarsh and mud flats, only 0.75 miles (1.21 km) across the water from the Isle of Wight. Temporary earthwork fortifications were erected on the site and, after the other three castles had been completed, work then began on Hurst in 1541 under the direction of John Mille, the financial controller, and probably Thomas Bertie, a master mason. Bertie was appointed as the castle's captain in 1542 and the work was completed by January 1544, at a cost of over £3,200.