Because of the novel explanations it generates for states' security- and identity-related behavior, the concept of ontological security has been used increasingly in the International Relations (IR) literature in recent years. However, the abundance of interpretations of the concept means that it is often used in conflicting ways. To counter the risk of conceptual stretching and provide the foundation for a common research agenda, this article constructs a typology of ontological security mechanisms. Two dimensions of ontological insecurity are highlighted: the sources and the causes of anxiety. We argue that the source of anxiety can be reflexive, relational, or systemic, while the cause of anxiety can be either shame or discontinuity. These two dimensions produce six mechanisms of ontological insecurity that reflect how the concept is used in the contemporary ontological security literature in IR. By specifying these mechanisms, we argue that the typology offers IR scholars the ability to produce even more nuanced and fine-grained explanations of state behavior driven by ontological insecurity. Finally, to demonstrate the utility of this typology, the article provides an illustrative case study of Russia's engagement in the conflict in Syria in 2015-2017.