At the beginning of the thirteenth century, following the Fourth Crusade and the collapse of the Byzantine Empire, especially in mainland and insular Greece, a series of principalities and lordships were formed, ruled by dynasties from Western Europe, mostly of French and Italian origin. These sovereigns, who were Catholics, reorganized the system of religious institutions, creating Catholic dioceses at the head of which bishops were often called from the territories from which the dynasties reigning originated. Thus, a peculiar situation of coexistence was created between diocesan institutions dependent on the Church of Rome and a population that instead followed the Orthodox confession for the most part. The present paper aims to provide an overview of the Catholic dioceses founded in the Crusader states of late medieval Greece, examining, where possible, the selection criteria and biographies of the bishops, their relations with the Crusader princes and the Greek population, and the ways in which the dioceses were ruled.