This article introduces an innovative theoretical conception of the corporate agency of international organizations (IOs). Existing rationalist and constructivist accounts attribute IO agency to the influence of intra-organizational agents. Drawing on general conceptions of corporate agency in International Relations, sociology, and philosophy, we elucidate how IOs can develop corporate agency, even if the member states prepare and adopt all organizational decisions themselves. In line with recent studies on international political authority, we replace the IO-as-bureaucracy model with the more comprehensive concept of IOs-as-governors. To establish the micro-foundations of IO agency, we adopt a bottom-up perspective and outline how, and under which conditions, IO agency arises from the interaction of constituent actors. Irrespective of any specific institutional design, IOs become actors in their own right whenever they gain action capability and autonomy. They acquire action capability whenever their members pool governance resources like the right to regulate certain activities or to manage common funds and authorize IOs to deploy these resources. IOs gain autonomy whenever they affect organizational decisions. Both dimensions of IO agency are variable and open to empirical enquiry. To illustrate our argument, we refer to the United Nations Security Council and other IOs with member-driven decision processes.