Neo-medievalism (or neomedievalism, new medievalism) is a term with a long history that has acquired specific technical senses in two branches of scholarship. In political theory about modern international relations, the term is originally associated with Hedley Bull. Political theory sees the political order of a globalized world as analogous to high-medieval Europe, in which neither states, nor the Church, nor other territorial powers, exercised full sovereignty. Instead, the institutions participated in complex, overlapping, and incomplete sovereignties.

In literary theory regarding the use and abuse of texts and tropes from the Middle Ages in postmodernity, the term neomedieval was popularized by the Italian medievalist Umberto Eco in his 1983 essay "Dreaming of the Middle Ages".

The idea of neomedievalism in political theory was first discussed in 1977 by theorist Hedley Bull in The Anarchical Society: A Study of Order in World Politics to describe the erosion of state sovereignty in the contemporary globalized world:

It is also conceivable that sovereign states might disappear and be replaced not by a world government but by a modern and secular equivalent of the kind of universal political organisation that existed in Western Christendom in the Middle Ages. In that system no ruler or state was sovereign in the sense of being supreme over a given territory and a given segment of the Christian population; each had to share authority with vassals beneath, and with the Pope and (in Germany and Italy) the Holy Roman Emperor above. The universal political order of Western Christendom represents an alternative to the system of states which does not yet embody universal government.

Thus Bull suggested society might move towards "a new mediaevalism" or a "neo-mediaeval form of universal political order", in which individual notions of rights and a growing sense of a "world common good" were undermining national sovereignty. He proposed that such a system might help "avoid the classic dangers of the system of sovereign states by a structure of overlapping structures and cross-cutting loyalties that hold all peoples together in a universal society while at the same time avoiding the concentration inherent in a world government", though "if it were anything like the precedent of Western Christendom, it would contain more ubiquitous and continuous violence and insecurity than does the modern states system".

In this reading, globalization has resulted in an international system which resembles the medieval one, where political authority was exercised by a range of non-territorial and overlapping agents, such as religious bodies, principalities, empires and city-states, instead of by a single political authority in the form of a state which has complete sovereignty over its territory. Comparable processes characterising Bull's "new medievalism" include the increasing powers held by regional organisations such as the European Union, as well as the spread of sub-national and devolved governments, such as those of Scotland and Catalonia. These challenge the exclusive authority of the state. Private military companies, multinational corporations and the resurgence of worldwide religious movements (e.g. political Islam) similarly indicate a reduction in the role of the state and a decentralisation of power and authority.

Stephen J. Kobrin in 1998 added the forces of the digital world economy to the picture of neomedievalism. In an article entitled "Back to the Future: Neomedievalism and the Postmodern Digital World Economy" in the Journal of International Affairs, he argued that the sovereign state as we know it – defined within certain territorial borders – is about to change profoundly, if not to wither away, due in part to the digital world economy created by the Internet, suggesting that cyberspace is a trans-territorial domain operating outside of the jurisdiction of national law.

Anthony Clark Arend also argued in his 1999 book Legal Rules and International Society that the international system is moving toward a "neo-medieval" system. He claimed that the trends that Bull noted in 1977 had become even more pronounced by the end of the twentieth century. Arend argues that the emergence of a "neo-medieval" system would have profound implications for the creation and operation of international law.