Pevensey Castle is a medieval castle and former Roman Saxon Shore fort at Pevensey in the English county of East Sussex. The site is a scheduled monument in the care of English Heritage and is open to visitors. Built around 290 AD and known to the Romans as Anderitum, the fort appears to have been the base for a fleet called the Classis Anderidaensis. The reasons for its construction are unclear; long thought to have been part of a Roman defensive system to guard the British and Gallic coasts against Saxon pirates, it has more recently been suggested that Anderitum and the other Saxon Shore forts were built by a usurper in an ultimately unsuccessful attempt to prevent Rome from reimposing its control over Britain.

Anderitum fell into ruin following the end of the Roman occupation but was reoccupied in 1066 by the Normans, for whom it became a key strategic bulwark. A stone keep and fortification was built within the Roman walls and faced several sieges. Although its garrison was twice starved into surrender, it was never successfully stormed. The castle was occupied more or less continuously until the 16th century, apart from a possible break in the early 13th century when it was slighted during the First Barons' War. It had been abandoned again by the late 16th century and remained a crumbling, partly overgrown ruin until it was acquired by the state in 1925.

Pevensey Castle was reoccupied between 1940 and 1945, during the Second World War, when it was garrisoned by units from the Home Guard, the British and Canadian armies and the United States Army Air Corps. Machine-gun posts were built into the Roman and medieval walls to control the flat land around Pevensey and guard against the threat of a German invasion. They were left in place after the war and can still be seen today. Pevensey is one of many Norman castles built around the south of England.

Pevensey Castle was constructed by the Romans on a spur of sand and clay that stands about 10 metres (33 ft) above sea level. In Roman times this spur was a peninsula that projected into a tidal lagoon and marshes, making it a strong natural defensive position. A harbour is thought to have been situated near the south wall of the castle, sheltered by a long spit of shingle where the village of Pevensey Bay stands now. A small river, Pevensey Haven, runs along the north side of the peninsula and would originally have discharged into the lagoon, but is now largely silted up.

Since Roman times, silting and land reclamation in the Pevensey Levels have pushed the coastline out by about 1.5 kilometres (0.9 mi), leaving the castle landlocked. The land between the castle and the sea is now a flat marshland drained by a network of ditches and sewers or field drains. The modern village of Pevensey is situated mostly to the east of the castle, at the end of the ancient peninsula. Castle Road (the B2191) curves around the Roman north wall and connects Pevensey to the nearby village of Westham. A public footpath crosses the interior of the castle, linking the two villages. An area of reclaimed land, formerly part of the Pevensey tidal lagoon but now marshland and fields crossed by the Eastbourne to Hastings railway line, is situated immediately to the south of the castle.

The castle occupies an area of about 3.67 hectares (9.1 acres). It has an oval plan on a north-east/south-west alignment, measuring 290 metres (950 ft) by 170 metres (560 ft). Not only is it the largest of the nine Saxon Shore forts, but its walls and towers are the largest of any surviving Roman fort of the period. Its shape is unique among Saxon Shore forts and was presumably determined by the contours of the peninsula on which it stands.

The castle's curtain wall is built on a massive scale, with ramparts and projecting towers still standing up to 8.2 metres (27 ft) high (and probably about 9.5 metres (31 ft) high when built) and 4.2 metres (14 ft) thick at the base. The north, east and west sections of the curtain wall have survived mostly intact, with the exception of one fallen segment of the north wall; the south wall, which would have adjoined sea or marsh, has almost entirely disappeared. It is faced with ironstone and sandstone, though most of the original facing stones have been robbed out over the centuries; the structure visible now consists mostly of the rubble and sandstone core, bound together with mortar. Bonding courses of tiles run horizontally through the wall. An impression of its original appearance can be gleaned from an area in the north wall which has been excavated down to the still-intact foundations, revealing how it was once faced on both sides with small blocks of stone. The wall originally had a stepped appearance with at least two levels of steps on the interior face, though there is no surviving indication of how the garrison reached the top. At the top of the wall the remains of medieval crenellations can still be seen, which probably replaced Roman originals.

The D-shaped towers along the curtain wall are similar to those of several other Saxon Shore forts, although their placement is somewhat unusual. Because the fort was partly surrounded by marshes and water, which provided natural defences, the Romans economised by only building towers on the more vulnerable north-eastern and far western sectors. The towers were probably used to mount artillery weapons such as catapults and heavy crossbows. Ten towers still survive, though there may originally have been more before the loss of the south wall.