Robert of Nantes was Latin patriarch of Jerusalem from 1240 to 1254, and, according to Bernard Hamilton, was “the most important single person” in the Frankish Kingdom of Jerusalem after the Battle of Forbie in 1244. Despite this importance, he was a rather obscure figure: almost nothing is known about him before he became bishop of Nantes in 1236. How did he rise to such a prominent position in Jerusalem? Robert of Nantes, Patriarch of Jerusalem (1240–1254) follows Robert from his probable origins in Aquitaine, to Italy where he might have been the unnamed bishop of Aquino. He was briefly transferred to Nantes in the duchy of Brittany, but soon returned to Rome, where he was appointed patriarch of Jerusalem in 1240. As patriarch, he was present for the fall of Jerusalem to the Khwarizmian Turks, the Frankish defeat at Forbie, and the subsequent crusade of Louis IX of France. This is the first book-length biography of any of the Latin patriarchs of Jerusalem. It will be of interest not only to historians of the crusades but also to historians of Italy, Sicily, the Papal States, the Holy Roman Empire, Aquitaine and Brittany. It will hopefully inspire further research on other ecclesiastical and secular leaders of Jerusalem and Cyprus, who may not be traditionally considered “rulers”, but who nevertheless helped govern the Frankish kingdoms.