Rollo lays siege to Chartres.

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

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.

The siege of Chartres took place in spring 911 during the age of Viking incursions in Europe. The Viking leader, Rollo, and his men laid siege to the city of Chartres, in West Francia. But they failed to achieve their goal before the arrival, in July 911, of a relief army sent to battle them. The engagement that ensued resulted in the defeat of the Norsemen. Although, Viking raids were far from being a rare occurrence as the Frankish realms had to contend with such sudden incursions for decades, this one would prove to have everlasting consequences on European history.