Portland Castle is an artillery fort constructed by Henry VIII on the Isle of Portland, Dorset, between 1539 and 1541. It formed part of the King's Device programme to protect against invasion from France and the Holy Roman Empire, and defended the Portland Roads anchorage. The fan-shaped castle was built from Portland stone, with a curved central tower and a gun battery, flanked by two angular wings. Shortly after its construction it was armed with eleven artillery pieces, intended for use against enemy shipping, operating in partnership with its sister castle of Sandsfoot on the other side of the anchorage. During the English Civil War, Portland was taken by the Royalist supporters of King Charles I, and then survived two sieges before finally surrendering to Parliament in 1646.

Portland continued to be used as a fort until the end of the Napoleonic Wars in 1815, when it was converted into a private house. Fresh concerns over invasion led to the War Office taking it over once again in 1869, but the castle was not rearmed and was instead formed as accommodation for more modern neighbouring fortifications. During the First and Second World Wars it was used as offices, accommodation and as an ordnance store. In 1949, the War Office relinquished control, and in 1955 it was opened to the public by the state. In the 21st century it is managed by English Heritage and operated as a tourist attraction, receiving 22,207 visitors in 2010. Historic England consider the castle to form "one of the best preserved and best known examples" of King Henry's forts.

Portland 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 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. Basic 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 in order 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 he 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.

As a consequence, Lord Russell inspected the coast along an anchorage known as Portland Roads, and concluded that two castles, Portland and Sandsfoot, should be constructed to protect it from naval attack. Work began that summer, with Oliver Lawrence acting as the royal paymaster for the project, and was completed by 1541, at a cost of £4,964. Thomas Mervin was appointed as captain, with a garrison of four gunners and two other men. In 1545, Mervin was replaced by John Leweston, who also took on responsibilities as the Lieutenant of Portland, along with a larger garrison of 13 men, paid for out of the proceeds of the recent Dissolution of the Monasteries. A survey between 1547 and 1548 reported that the castle was equipped with one brass demi-cannon, two brass demi-culverins, four breech-loading portpieces and four slings; it also held eight hagbushes–a type of early arquebus–along with twenty-three bows, twenty-nine bills and twelve pikes.

The threat of a French invasion passed, and peace was declared in 1558. Attention shifted away from Portland, and a 1574 survey reported that the castle was in a poor condition, with similar concerns being repeated in 1583. Tensions with Spain grew and military attention focused on the threat to the south-west of England. War broke out in 1585, and £228 was spent on renovating Portland Castle. In 1596, the garrison still comprised a captain and 13 men.

In the early 1600s, England was at peace with France and Spain and the coastal forts, included Portland, received little attention. A 1623 survey reported that the castle was equipped with three culverins, nine demi-culverins and a saker, but that the fortifications had suffered badly from sea erosion and required extensive repairs. Fourteen years later, the castle had 15 guns and a garrison of a captain and 12 men.

When the English Civil War broke out in 1642, Portland was initially controlled by Parliamentary forces. The castle was captured in 1643 by a group of Royalists who gained access by pretending to be Parliamentary soldiers. As the war turned against the King in the south-west, Parliamentary forces besieged the castle for four months in 1644, and once again the following year. The castle finally surrendered to Vice-Admiral William Batten in April 1646. It is uncertain why the castle, which was not easily defensible on the landward side, proved so difficult to take; the historian Peter Harrington has suggested that its low-lying position may have made it difficult for Parliament to bombard it from the sea.