Signing of the Treaty of Saint-Clair-sur-Epte between Charles the Simple and Rollo of Normandy.

911 (CMXI) was a common year starting on Tuesday of the Julian calendar.

The treaty of Saint-Clair-sur-Epte (911) is the foundational document of the Duchy of Normandy, establishing Rollo, a Norse warlord and Viking leader, as the first Duke of Normandy in exchange for his loyalty to Charles III, the king of West Francia, following the Siege of Chartres. This treaty included a land grant of farmable coastal land from Charles the Simple to Rollo in return for his fealty and protection against other Viking groups that had been regularly raiding the northern coastline. This treaty effectively changed how mainland Europe knew to resolve ongoing raids from the Vikings.

Charles III, called the Simple or the Straightforward, was the king of West Francia from 898 until 922 and the king of Lotharingia from 911 until 919–923. He was a member of the Carolingian dynasty.

Rollo, also known with his epithet, Rollo "the Walker", was a Viking who, as Count of Rouen, became the first ruler of Normandy, a region in today's northern France. He was prominent among the Vikings who besieged Paris in 885–886, and he emerged as a war leader among the Norsemen who had secured a permanent foothold on Frankish soil in the valley of the lower Seine after the Siege of Chartres in 911. Charles the Simple, king of West Francia, agreed to the Treaty of Saint-Clair-sur-Epte, which granted Rollo lands between the river Epte and the sea in exchange for Rollo agreeing to end his brigandage, swear allegiance to Charles, convert to Christianity, and pledge to defend the Seine estuary from other Viking raiders.