Wallingford Castle is a medieval castle situated in Wallingford in the English county of Oxfordshire (historically Berkshire), adjacent to the River Thames. Established in the 11th century as a motte-and-bailey design within an Anglo-Saxon burgh, it grew to become what historian Nicholas Brooks has described as "one of the most powerful royal castles of the 12th and 13th centuries". Held for the Empress Matilda during the civil war years of the Anarchy, it survived multiple sieges and was never taken. Over the next two centuries it became a luxurious castle, used by royalty and their immediate family. After being abandoned as a royal residence by Henry VIII, the castle fell into decline. Refortified during the English Civil War, it was eventually slighted, i.e. deliberately destroyed, after being captured by Parliamentary forces after a long siege. The site was subsequently left relatively undeveloped, and the limited remains of the castle walls and the considerable earthworks are now open to the public.

Prosperous with its own mint and an important regional town overlooking a key crossing point on the River Thames, the town of Wallingford had been defended by an Anglo-Saxon burgh, or town wall, prior to the Norman invasion of 1066. Wigod of Wallingford, who controlled the town, supported William the Conqueror's invasion and entertained the king when he arrived in Wallingford. Immediately after the end of the initial invasion, the king set about establishing control over the Thames Valley through constructing three key castles, the royal castles of Windsor and Wallingford, and the baronial castle, later transferred to royal hands, built at Oxford.

Wallingford Castle was probably built by Robert D'Oyly between 1067 and 1071. Robert had married Wigod's daughter Ealdgyth, and ultimately inherited many of his father-in-law's lands. The wooden castle was built in the north-east corner of the town, taking advantage of the old Anglo-Saxon ramparts, with the motte close to the river overlooking the ford, and required substantial demolition work to make room for the new motte-and-bailey structure. Unusually, it appears that the castle was constructed on top of high-status Anglo-Saxon housing, probably belonging to former housecarls. The motte today is 60 metres (197 feet) across and 13 metres (43 feet) high. Robert endowed a sixteen-strong college of priests within the castle, which he named St Nicholas College.

Wallingford Castle passed from Robert to first his son-in-law Miles Crispin, and then Brien FitzCount, who married Robert's daughter, Matilda, after Miles died. Brien, an important supporter of Henry I, was the son of the Duke of Brittany, and strengthened the castle in stone, probably in the 1130s. He produced a very powerful fortification, including a shell keep and a curtain wall around the bailey, that, combined with the extensive earthworks, has been described by historian Nicholas Brooks as "one of the most powerful royal castles of the 12th and 13th centuries".

After the death of Henry, however, the political situation in England became less stable, with both Stephen of England and the Empress Matilda laying claim to the throne. Brien had originally been considered a supporter of Stephen, but in 1139 Matilda travelled to England and Brien announced his allegiance to her, joining forces with Miles of Gloucester and other supporters in the south-west. Wallingford Castle was now the most easterly stronghold of the Empress's faction – it was either the closest base to London, or the first in line to be attacked by Stephen's forces, depending on one's perspective.

Stephen attacked the castle in 1139, initially intending to besiege it, as the walls were considered impregnable to assault. Brien had brought in considerable supplies – contemporaries believed the castle could survive a siege for several years if need be – and Stephen changed his mind, putting up two counter-castles to contain Wallingford along the road to Bristol, before continuing west. The next year, Miles of Gloucester, possibly acting under orders from Robert of Gloucester, struck east, destroying one of the counter-castles outside Wallingford. The civil war between Stephen and Matilda rapidly descended into an attritional campaign, in which castles like Wallingford played a critical role in efforts by both sides to secure the Thames Valley. After the fall of Oxford to Stephen in 1141, Matilda fled to Wallingford, and the importance of the castle continued to grow.

Around this time Brien established a notorious prison within the castle, called Cloere Brien, or "Brien's Close", as part of his efforts to extract money and resources from the surrounding region. The nobleman William Martel, Stephen's royal steward, was one of the most high-profile prisoners to be kept there. Contemporary chroniclers reported the cries of tortured prisoners in the castle disturbed the inhabitants of the town of Wallingford. There was not enough space in the castle for all of Brien's forces, and various houses in the town had to be taken for the use of his knights.

Between 1145 and 1146 Stephen made another attempt to seize Wallingford, but was again unable to take the castle despite building a powerful counter-castle to the east, opposite Wallingford at Crowmarsh Gifford, and building castles to the west at Brightwell, South Moreton and Cholsey. He returned with larger forces in 1152, reestablishing the counter-castle at Crowmarsh Gifford and building another one overlooking Wallingford bridge, and settled his forces down to starve the castle out. Brien, supported by Miles' son, Roger of Hereford, who had also become trapped in the castle, attempted to break through the blockade, but without success.