Theorizing about the manner in which urban dimensions influence global security and vice versa is still in an embryonic stage. The central argument of this article holds that scholars in the fields of international relations (IR) and security studies largely remain blind to contemporary urban dimensions of global transboundary security issues, and have not yet adequately assessed its dynamics or political implications. In order to stimulate and structure further research, this article sets out to situate issues pertaining to urban security within a wider IR theoretical context. I suggest that the contemporary global security environment can be conceptualized in terms of a global–urban security nexus. This nexus points, on the one hand, to a changing spatial dynamic of security where urban places and practices become increasingly imperative and, on the other hand, to a rescaling of state power where urban actors are becoming increasingly empowered. I suggest that the global–urban security nexus as a point of analytical departure is equally relevant to the more traditional, narrow understanding of security as it is to the broadened security agenda, and it captures contemporary spatial security dynamics as well as changing security governance, in terms of both involved actors and practices. I finally draw out an urban security research agenda for IR that puts focus on global and transboundary security problems and their urban facets, and offers a novel way forward for studying global security dynamics in terms of its urban spaces, agents, and practices.