The objective of this article is to explore the spatial and temporal dynamics of an important but often neglected space of global governance: the cities in which the headquarters of international organizations (IOs) are located. For this purpose, this article proposes a conceptualization and an empirical application of the concept of ‘ecosystem’. This conceptualization builds on classic sociological ideas and organization theory to develop an innovative understanding of these cities which are more than mere hubs. We use this metaphor to describe an HQ city where one or several IOs have their seats. As a result, it is a space characterized by specific geographical and temporal features that can be qualified as spatial and temporal proximity between the elements composing the ecosystem. Based on original empirical sources, we apply this concept to the so-called International Geneva. We argue that conceptualizing headquarters as ecosystems helps to consider how HQs’ location influences the daily work of IOs.