Sovereignty is the core concept of international relations. Almost without exception, approaches to sovereignty in IR have followed a binary framing where sovereignty is seen to consist of two components: ‘internal’ versus ‘external’ sovereignty, ‘positive’ versus ‘negative’ sovereignty, and so on. These dichotomies stem from the prevailing understanding of sovereignty as the boundary between the inside and the outside of the state. This article builds on and expands these existing approaches by reconceptualizing the sovereign border line as a liminal border space. Relatedly, we theorize the concept of liminality in greater depth by distinguishing between four distinct kinds of liminality: marginal, hybrid, interstitial, and external. Each of these problematizes the dividing line of sovereignty in unique but comparable ways. We empirically illustrate these four kinds of liminality with reference to contested states, ‘tribal’ or ‘indigenous’ groups, NGOs such as Amnesty International, and extremist groups such as ISIS, respectively. Each of these types of liminality entails unique actors, practices, and consequences for the concept of sovereignty. We suggest that liminal sovereignty practices represent the most radical source of change for the concept of sovereignty, yet at the same time, somewhat counterintuitively, they also serve as the best means of clarifying existing, established meanings and practices of sovereignty.