Robert Curthose (c.1051 – 3 February 1134), sometimes called Robert II, succeeded his father, William the Conqueror as Duke of Normandy in 1087 and reigned until 1106. Robert was also an unsuccessful claimant to the throne of the Kingdom of England. The epithet "Curthose" had its origins in the Norman French word courtheuse "short stockings" and was apparently derived from a nickname given to Robert by his father; the chroniclers William of Malmesbury and Orderic Vitalis reported that William the Conqueror had derisively called Robert brevis-ocrea ("short boot").

The eldest son of William the Conqueror, Robert's reign as duke is noted for the discord with his brothers William II and Henry I in England. Robert mortgaged his duchy to finance his participation in the First Crusade, where he was an important crusader commander. Eventually, his disagreements with Henry I led to his death in captivity and the absorption of Normandy as a possession of England.

In 1087, the elder William died of wounds suffered from a riding accident during a siege of Mantes. At his death he reportedly wanted to disinherit his eldest son but was persuaded to divide the Norman dominions between his two eldest sons. To Robert he granted the Duchy of Normandy and to William Rufus he granted the Kingdom of England. The youngest son Henry was given money to buy land. Of the two elder sons Robert was considered to be much the weaker and was generally preferred by the nobles who held lands on both sides of the English Channel since they could more easily circumvent his authority.

At the time of their father's death the two brothers made an agreement to be each other's heir. However this peace lasted less than a year when barons joined with Robert to displace Rufus in the Rebellion of 1088. It was not a success, in part because Robert never showed up to support the English rebels.

Robert took as his close adviser Ranulf Flambard, who had been previously a close adviser to his father. Flambard later became an astute but much-disliked financial adviser to William Rufus until the latter's death in 1100.

In 1096, Robert formed an army and left for the Holy Land on the First Crusade. At the time of his departure he was reportedly so poor that he often had to stay in bed for lack of clothes. To raise money for the crusade he mortgaged his duchy to his brother William for the sum of 10,000 marks.

When William II died on 2 August 1100, Robert was on his return journey from the Crusade and was about to marry a wealthy young bride to raise funds to buy back his duchy. As a result of Robert's absence, his brother Henry was able to seize the crown of England for himself.

Upon his return, Robert – urged by Flambard and several Anglo-Norman barons – claimed the English crown, on the basis of the short-lived agreement of 1087, and in 1101 led an invasion to oust his brother Henry. He landed at Portsmouth with his army, but the lack of popular support among the English (Anselm, the archbishop of Canterbury, was decidedly against him and the Charter of Liberties issued at Henry's coronation was well liked) as well as Robert's own mishandling of the invasion tactics enabled Henry to resist the invasion. Robert was forced by diplomacy to renounce his claim to the English throne in the Treaty of Alton. It is said that Robert was a brilliant field commander but a terrible strategist in the First Crusade.