The present dissertation aims to rewrite the history of ideas about the ancient notion of tyranny in the early medieval Frankish world. It focuses in particular on how members and allies of the Carolingian dynasty craftily manipulated this term in historiographical and hagiographical texts in order to legitimize their power, to justify acts of violence, or to demonize enemies inside of and beyond the borders of their empire. Tyranny has been a topic of discussion among historians of medieval political thought for over a century. Scholarly understandings of this concept's evolution during the formative period of the Early Middle Ages, however, have long been skewed by assumptions about an apparent Patristic monopoly on ideas about tyranny in the Latin West until the rise of "juridical" definitions following the rediscovery of Roman law and the emergence of Aristotelian Scholasticism in the High Middle Ages. In response to these misguided preoccupations, "Tyrannus Rex" shifts its focus to a substantial body of historiographical and hagiographical texts traditionally undervalued by historians of political thought in order to trace the trajectory of discussions about tyranny in the Frankish world between the fifth and ninth centuries. As a result, this dissertation offers three significant contributions to discussions of medieval ideas about tyranny and Carolingian political history: firstly, that early medieval thinkers did not simply parrot earlier ideas about tyranny but rather creatively reshaped this concept for persuasive purposes in their specific sociopolitical contexts; secondly, that the manipulation of concepts and terminology like "tyrannus" in texts composed in support of the Carolingian dynasty constitutes an important dynastic power practice in keeping with others—capitularies, charters, shows of force, etc.—discussed more broadly by scholars of Carolingian history; and, finally, that the involvement of members and allies of the dynasty in the production and dissemination of persuasive, pro-dynastic representations of tyranny constitutes a kind of propaganda that served to advance or to reinforce the dynasty's claims to authority over a polity which they had usurped.