The Treaty of Alton was an agreement signed in 1101 between Henry I of England and his older brother Robert, Duke of Normandy in which Robert agreed to recognize Henry as king of England in exchange for a yearly stipend and other concessions. The agreement temporarily ended a crisis in the succession of the Anglo-Norman kings.
The previous year, Henry had seized the throne on the death of his brother William II, even though Robert had been William's acknowledged heir. Robert's absence in the First Crusade at the time of William's death, as well as his poor reputation among the Anglo-Norman barons allowed Henry to claim the throne with popular support.
Robert returned from the Holy Land after Henry's coronation, and encouraged by his advisor Ranulf Flambard, he invaded the Kingdom of England in order to claim the throne. He landed secretly at Portsmouth and was met by Henry at the town of Alton.
The dispute was settled diplomatically. In the treaty, Robert agreed to renounce his claim to the English throne in exchange for a yearly stipend of 3000 marks and the concession of all but one of Henry's possessions in the Duchy of Normandy. Robert and his followers were allowed to return to Normandy without reprisal from Henry. The two brothers moreover agreed to name each other as heirs and to assist each other in the punishment of traitors.
The agreement was not long-lasting, however. In 1105, Henry invaded Normandy and defeated his brother's army the following year at the Battle of Tinchebray. Robert was imprisoned and died in captivity in 1134. Normandy remained a possession of the English crown for over a century afterwards.