Portchester Castle is a medieval fortress that was developed within the walls of the Roman Saxon Shore fort of Portus Adurni at Portchester, to the east of Fareham in Hampshire.

The keep was probably built in the late 11th century as a baronial castle, before Portchester came under royal control in 1154. The monarchy controlled the castle for several centuries; it was a favoured hunting lodge of King John, and was besieged and captured by the French in 1216 before permanently returning to the English shortly thereafter.

With a commanding position at the head of Portsmouth Harbour, Portchester was an important port in the medieval period. The castle saw the embarkation for several campaigns to France led by England's kings. In anticipation of a French invasion during the first quarter of the 14th century, Edward II spent £1,100 repairing and reinforcing Portchester Castle. A plot to overthrow Henry V was discovered and the culprits apprehended at Portchester; this event features in Shakespeare's play Henry V. Later in its history, the castle was used as a prison.

Today Portchester Castle is a Scheduled monument, and a Grade I listed building. The castle has been in the ownership of the Southwick Estate since the 17th century but is managed by English Heritage and open to visitors throughout the year. The Norman church, St. Mary's, which stands in the south-east corner of the grounds, falls within the Anglican Diocese of Portsmouth.

The strategic importance of Portchester has been recognised since at least the 3rd century when a Roman fort was established on the site. Though it is uncertain exactly when the fort was constructed, it is thought that it was built by Marcus Aurelius Carausius on the instructions of emperor Diocletian between 285 and 290. It was one of several forts built along the British coast in the period to combat raids by pirates. Portchester was probably a base from which the Classis Britannica, the Roman fleet defending Britain, operated. It is the best preserved Roman fort north of the Alps.

Although the Roman army retreated from Britain in the early 5th century, it is unlikely that the fort was ever completely abandoned, although its use continued on a much smaller scale. A 10th-century hall and tower were discovered within the fort, suggesting it was a high-status residence during the Saxon period. In 904, Portchester came into the possession of King Edward the Elder and the fort became a burh to help defend the country against Vikings, as listed in the Burghal Hidage.

It is uncertain when the castle was built, although it was probably in the late 11th century. In the aftermath of the Norman Conquest, the manor of Portchester was granted to William Maudit, an associate of William the Conqueror and a powerful magnate and it was probably he who built Portchester Castle.

The form of this early castle is uncertain, although Maudit was probably responsible for creating the inner ward in the north-west corner of the fort. At this point it would probably have been defended by a wooden palisade and a moat, with the original Roman stone walls of the fort acting as the defence of the outer bailey. Maudit died in about 1100, and his property passed onto his son, Robert Maudit. He died in 1120, and a few years later the family estates came into the hands of William Pont de l'Arche through marriage to Robert Maudit's daughter.