Sandown Castle was an artillery fort constructed by Henry VIII in Sandown, Kent, between 1539 and 1540. It formed part of the King's Device programme to protect against invasion from France and the Holy Roman Empire, and defended the strategically important Downs anchorage off the English coast.

Comprising a keep and four circular bastions, the moated stone castle covered 0.59 acres (0.24 ha) and had 39 firing positions on the upper levels for artillery, with 31 gunloops in the basement for handguns. It cost the Crown a total of £27,092 to build the three castles of Sandown, Walmer and Deal, which lay adjacent to one another along the coast and were connected by earthwork defences. The original invasion threat passed, but during the Second English Civil War of 1648–49, Sandown was seized by pro-Royalist insurgents and was only retaken by Parliamentary forces after several months' fighting.

By the 19th century, the castle was suffering badly from the effects of coastal erosion but remained in military use until 1863. The War Office then demolished the upper levels with explosives, carrying out a second wave of demolition work in 1882 and destroying most of the surviving stonework in 1893. The remains of Sandown were purchased by the town of Deal for £35 to form part of the local sea defences. The remaining masonry was encased in concrete in the late 1980s to form a sea wall but remains vulnerable to further erosion by the sea.

Sandown 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 the local lords and communities, only taking a modest role in building and maintaining fortifications, and while France and the Empire remained in conflict with one another, 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 very limited in scale.

In 1533, Henry broke with Pope Paul III to annul the long-standing marriage to his wife, Catherine of Aragon, and remarry. Catherine was the aunt of Charles V, the Holy Roman Emperor, and Charles 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.

Sandown and the adjacent castles of Deal and Walmer were constructed to protect the Downs in east Kent, an important anchorage formed by the Goodwin Sands that gave access to Deal Beach, on which enemy soldiers could easily be landed. The stone castles were supported by a line of four earthwork forts, known as the Great Turf, the Little Turf Bulwark, the Great White Bulwark of Clay and the Walmer Bulwark, and a 2.5 miles (4.0 km) long defensive ditch and bank. Collectively the castles became known as the "castles of the Downs" and cost the Crown a total of £27,092.

The castle was built between April 1539 and the autumn of 1540, by a team including Richard Benese as the surveyor, William Clement as the master carpenter, and Christopher Dickenson as the master mason. It was nearly identical to its sister castle at Walmer, with a tall keep, 83 feet (25 m) across, at the centre, and flanked by four rounded bastions, also called lunettes, and a moat, which was surrounded in turn by a curtain wall. Its curved walls were 15 feet (4.6 m) thick. In total it was approximately 165 by 165 feet (50 by 50 m) across, covering 0.59 acres (0.24 ha). The historian John Hale considered the castle to form a transitional design between older medieval English designs and newer Italian styles of defence.

Sandown had three tiers of artillery – the heaviest and longest range weapons occupying the upper levels – with a total of 39 firing positions, and 31 gunloops in the basement for handguns should close defence be required. The embrasures in the walls were all widely splayed to provide the maximum possible space for the guns to operate and traverse, and the interior of the castle was designed with vents to allow the smoke from its guns to escape. It was initially garrisoned by a captain, two lieutenants, two porters, ten gunners and three soldiers, at an annual cost of £174 a year.