Pevensey (/ˈpɛvənzi/ PEV-ən-zee) is a village and civil parish in the Wealden district of East Sussex, England. The main village is located five miles (8 km) north-east of Eastbourne, one mile (1.6 km) inland from Pevensey Bay. The settlement of Pevensey Bay forms part of the parish. It was here that William the Conqueror made the landing in his invasion of England in 1066 after crossing the English Channel from Normandy.

Pevensey is situated on a spur of sand and clay, about 33 feet (10 m) above sea level. In Roman times this spur was a peninsula that projected into a tidal lagoon and marshes. 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. The lagoon extended inland as far north as Hailsham and eastwards to Hooe. With the effect of longshore drift this large bay was gradually cut off from the sea by shingle, so that today's marshes are all that remain behind the shingle beach.

The marshes, known as the Pevensey Levels, cover an area of around 47 square miles (120 km²). The marshes are a Site of Special Scientific Interest and a large nature reserve, jointly owned by Natural England and the Sussex Wildlife Trust. There are many nationally rare plants and invertebrates, including the fen raft spider. The site is very fragile and general access is not permitted.

Pevensey is also the start point of the 1066 Country Walk which is a long-distance footpath covering many historical sites in the area.

The settlement of Pevensey Bay lies behind and on the shingle beach. Although small, it is nevertheless a seaside resort in miniature and has many of the facilities of its larger counterparts elsewhere. It is a clay bay, making it susceptible to erosion over time. The shingle beach at Pevensey Bay provides an important defence against flooding and storm damage from the sea for a large area of low-lying land beyond. There are two Martello towers, constructed in 1806 as Napoleonic coastal defences. Before development commenced it was known as Wallsend; the 16th century Castle Hotel standing alone on the beach.

The earliest evidence for the name Pevensey is in later copies of charters dated to 788 and 790, and the name occurs in a variety of forms, including Pefensea, Pæfensea and Pævenisel. The name means "River of [a man named] Pefen" and derives from the Anglo-Saxon personal name Pefen plus eã, "river", presumably a reference to the now largely silted-up Pevensey Haven. The spellings Pemse and Pemsey indicate the local pronunciation [pimzi], [pemzi].

By the 4th century AD the south and east of the province of Britannia was under frequent attack from marauding barbarian tribes: including the Jutes and Saxons. To counter these attacks the Romans built a total of eleven forts between Essex and the Isle of Wight, now known as the Saxon Shore Forts. The fort at Pevensey, built between AD 300 and 340, was named Anderitum. The earliest stone remains on the site date from the Roman period, including the outer bailey wall. The sea washed over what is now Pevensey Marshes, surrounding the fort on three sides.

When the Roman army left Britain, the province was more vulnerable to attack, first by the Jutes in east Kent, and the Romanised native Britons attempted to defend their island from attack. Following the Jutish example the Saxons began invading Britain in earnest. Around 491, Saxons, possibly led by Ælle of Sussex began to colonise the south coast and besieged Anderitum over a number of years. After a long struggle the British defences were overrun. Some remaining Britons on the south coast fled north, others emigrated by boat to what is now called Brittany and the area became the Kingdom of the South Saxons, later called Sussex. The old Roman fort of Anderida was burned and left derelict. For a while the ruined castle was known by the Saxons as Andredceaster and the Weald of southern England – which stretches 120 miles (200 km) from Anderida to Dorset – was named Andredsweald, the Forest of Anderida.