The expanding reach of international investment law and the negotiation of major economic treaties between democratic polities have prompted new debates about the relationship between democracy and the international investment regime. This article develops an analytical framework for understanding that relationship. It first unpacks the concept of democracy, exploring the ‘rules-based’ and ‘action-based’ conceptions that emerge from political theory and their relevance to international investment law. It then examines three themes that frame the relationship between democracy and international investment law: the interface between the investment regime and national democratic space; the place of democratic processes in investment treaty making; and public participation in the settlement of investment disputes. The interplay between rules- and action-based dimensions provides a common thread across the three themes. The article concludes that there is a gap between formal rules and citizen action in promoting democratic oversight, and significant scope to develop more effective mechanisms to install democratic governance in the creation and implementation of international investment law.