Complex adaptive systems are a special kind of self-organizing system with emergent properties and adaptive capacity in response to changing external conditions. In this article, we investigate the proposition that international environmental law, as a network of treaties and institutions, exhibits some key characteristics of a complex adaptive system. This proposition is premised on the scientific understanding that the Earth system displays properties of a complex adaptive system. If so, international environmental law, as a control system, may benefit from the insights gained and from being modelled in ways more appropriately aligned with the functioning of the Earth system itself. In this exploratory review, we found evidence suggesting that international environmental law is a complex system where treaties and institutions self-organize and exhibit emergent properties. Furthermore, we contend that international environmental law as a whole is adapting to exogenous changes through an institutional process akin to natural selection in biological evolution. However, the adequacy of the direction and rate of adaptation for the purpose of safeguarding the integrity of Earth’s life-support system is questioned. This paper concludes with an emphasis on the need for system-level interventions to steer the direction of self-organization while maintaining institutional diversity. This recommendation stands in contrast to the reductionist approach to institutional fragmentation and aims at embracing the existing complexity in international environmental law.