It is time for International Relations (IR) to join the relational revolution afoot in the natural and social sciences. To do so, more careful reflection is needed on cosmological assumptions in the sciences and also in the study and practice of international relations. In particular it is argued here that we need to pay careful attention to whether and how we think 'relationally'. Building a conversation between relational cosmology, developed in natural sciences, and critical social theory, this book seeks to develop a new perspective on how to think relationally in and around the study of IR. International Relations in a Relational Universe asks: What kind of cosmological background assumptions do we make as we tackle international relations today and where do our assumptions (about states, individuals, or the international) come from? And can we reorient our cosmological imaginations towards more relational understanding of the universe and what would this mean for the study and practice of international politics? The book argues that we live in a world without 'things', a world of processes and relations. It also suggests that we live in relations which exceed the boundaries of the human and the social, in planetary relations with plants and animals. Rethinking conceptual premises of IR, Kurki points towards a 'planetary politics' perspective within which we can reimagine IR as a field of study and also political practices, including the future of democracy.