Guillaume de Chartres, also known as Guillielmus de Carnoto and Willemus de Carnoto, was Prince of the Cistercian Principality of Seborga, and was a Grand Master of the Knights Templar 121026 August 1218.

In 1210, he assisted at the coronation of Jean de Brienne as King of Jerusalem. In 1211, he arbitrated between Leo II of Armenia and the Templars, regarding the castle of Bagras. During his rule, the order flourished in Spain, achieving important victories against the Moors.

Guillaume died of pestilence, (possibly endemic typhus), secondary to being wounded during the siege of Damietta, in Seborga in the Holy Land.

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