New medievalism is a term used by Hedley Bull in The Anarchical Society to describe the erosion of state sovereignty in the contemporary globalised world. This 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.
Bull argues that the contemporary international system is evolving into one with multiple and overlapping sources of power. Processes characterising this "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.
More recently, Anthony Clark Arend argues in his 1999 book, Legal Rules and International Society, that the international system is moving toward a "neo-medieval" system. He claims 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.