The Treaty of Deabolis (Greek: συνθήκη της Δεαβόλεως) was an agreement made in 1108 between Bohemond I of Antioch and Byzantine Emperor Alexios I Komnenos, in the wake of the First Crusade. It is named after the Byzantine fortress of Deabolis (modern Devol, Albania). Although the treaty was not immediately enforced, it was intended to make the Principality of Antioch a vassal state of the Byzantine Empire.
At the beginning of the First Crusade, crusader armies assembled at Constantinople and promised to return to the Byzantine Empire any land they might conquer. However, Bohemond, the son of Alexios' former enemy Robert Guiscard, claimed Antioch for himself. Alexios did not recognize the legitimacy of the principality, and Bohemond went to Europe looking for reinforcements. He launched into open warfare against Alexios, laying siege to Dyrrhachium, but he was soon forced to surrender and negotiate with Alexios at the imperial camp at Deabolis, where the Treaty was signed.
Under the terms of the Treaty, Bohemond agreed to become a vassal of the emperor and to defend the Empire whenever needed. He also accepted the appointment of a Greek patriarch. In return, he was given the titles of sebastos and doux (duke) of Antioch, and he was guaranteed the right to pass on to his heirs the County of Edessa. Following this, Bohemond retreated to Apulia and died there. His nephew, Tancred, who was regent in Antioch, refused to accept the terms of the treaty. Antioch came temporarily under Byzantine sway in 1137, but it was not until 1158 that it truly became a Byzantine vassal.
The Treaty of Deabolis is viewed as a typical example of the Byzantine tendency to settle disputes through diplomacy rather than warfare, and was both a result of and a cause for the distrust between the Byzantines and their Western European neighbors.