Rochester Castle stands on the east bank of the River Medway in Rochester, Kent, South East England. The 12th-century keep or stone tower, which is the castle's most prominent feature, is one of the best preserved of its time in England or France.

Situated on the River Medway and Watling Street, Rochester was a strategically important royal castle. During the late medieval period, it helped protect England's south-east coast from invasion. The first castle at Rochester was founded in the aftermath of the Norman Conquest. It was given to Bishop Odo, probably by his half-brother William the Conqueror. During the Rebellion of 1088 over the succession to the English throne, Odo supported Robert Curthose, the Conqueror's eldest son, against William Rufus. It was during this conflict that the castle first saw military action; the city and castle were besieged after Odo made Rochester a headquarters for the rebellion. After the garrison capitulated, this first castle was abandoned.

Between 1087 and 1089, Rufus asked Gundulf, Bishop of Rochester, to build a new stone castle at Rochester. He established the current extent of the castle. Though much altered through the centuries, some parts of Gundulf's work survive. In 1127 King Henry I granted the castle to the Archbishop of Canterbury in perpetuity. William de Corbeil built the massive keep that still dominates the castle today. Throughout the 12th century the castle remained in the custody of the archbishops.

During the First Barons' War (1215–1217) in King John's reign, baronial forces captured the castle from Archbishop Stephen Langton and held it against the king, who then besieged it. The Barnwell chronicler remarked "Our age has not known a siege so hard pressed nor so strongly resisted". After resisting for just over seven weeks, the garrison surrendered. The castle had been greatly damaged, with breaches in the outer walls and one corner of the keep collapsed, and hunger eventually forced the defenders' hand. The castle did not stay under John's control for long: in 1216 it was captured by the French Prince Louis, who was the new leader of the baronial faction. John died and was succeeded by his son King Henry III in 1216; the next year, the war ended and the castle was taken under direct royal control.

Rochester was besieged for the third time in 1264 during the Second Barons' War (1264–1267). The castle's royal constable, Roger de Leybourne, held Rochester in support of Henry III. Rebel armies led by Simon de Montfort and Gilbert de Clare entered the city and set about trying to capture the castle. Again the castle's defenders resisted, this time with a different outcome. After a week, the rebel armies raised the siege in the face of relief from Henry himself. The garrison did not surrender, and the castle suffered extensive damage that was not repaired until the following century. The castle saw military action for the last time in 1381 when it was captured and ransacked during the Peasants' Revolt. As Rochester Castle fell out of use its materials were reused elsewhere and custodianship was relinquished by the Crown. The castle and its grounds were opened to the public in the 1870s as a park. At various points during the 19th and 20th  centuries repairs were carried out. The castle is protected as a Grade I listed building and Scheduled Monument. Today the ruins are in the guardianship of English Heritage and open to the public.

Castles were introduced to England by the Normans in the 11th century and their construction, in the wake of the conquest of 1066, helped the Normans secure their new territory. Rochester was an important city, built on the site of a Roman town at the junction of the River Medway and Watling Street, a Roman road. It has long been assumed that the first castle was located next to the river, just outside the south-west corner of the town walls. The conjectural site of the early castle later became known as "Boley Hill". Archaeologist Tom McNeill has suggested that these earliest castles in England may have been purely military, built to contain a large number of troops in hostile territory.

According to the Domesday Book of 1086, the Bishop of Rochester was given land valued at 17s 4d in Aylesford, Kent, in compensation for land that became the site of Rochester Castle. Of the 48  castles mentioned in the survey, Rochester is the only one for which property owners were reimbursed when their land was taken to build the castle. From the 11th century the castle-guard was a feudal obligation in England. This often took the form of knights garrisoning castles for their lords for a set period. There is no comprehensive list of which castles were owed service in this form, but military historian Cathcart King notes that they seem to have been predominantly high-status castles. Rochester's castle-guard consisted of 60 knights' fees, marking it as a particularly important fortification.

It was probably William the Conqueror who gave the city and its castle to Bishop Odo of Bayeux, the king's half-brother. On William's death in September 1087, his territories were divided between his two sons. Robert, the elder, inherited the title of Duke of Normandy and William Rufus became King of England. A significant number of Norman barons objected to dividing Normandy and England, and Bishop Odo supported Robert's claim to the English throne. Several others, including the earls of Northumberland and Shrewsbury and the Bishop of Coutances, came out in support of Robert. Odo prepared Rochester Castle for war and it became one of the headquarters of the rebellion. Its position in Kent made it a suitable base for raids on London and its garrison could harry William's forces in the county. William set off from London and marched towards Rochester to deal with the threat. Before he arrived, news reached the king that Odo had gone to Pevensey Castle, which was under the control of Robert, Count of Mortain. William turned away from Rochester and seized Pevensey. The captured Odo was forced to swear to hand over Rochester to William's men. The king despatched a force with Odo in tow to demand Rochester's surrender. Instead of yielding, the garrison sallied and captured the entire party. In response, William laid siege to the city and castle. Contemporary chronicler Orderic Vitalis recorded that the siege began in May 1088. Two siege-castles were built to cut off the city's supply lines and to protect the besiegers from sorties. Conditions within the city were dire: disease was rampant, exacerbated by the heat and flies. The garrison ultimately capitulated and terms were agreed. Odo, Eustace, Count of Boulogne, and Robert of Bellême, son of the Earl of Shrewsbury, were allowed to march away with their weapons and horses but their estates in England were confiscated. This marked the end of the castle's role in the rebellion, and the fortification was probably abandoned shortly afterward. The siege castles were abandoned after the conclusion of the siege and have since vanished.