While the growing trend towards constitutional enactment of environmental rights has mainly been discussed in normative and descriptive terms, few scholars have endeavoured to explain the phenomenon in a systematic fashion and none have approached the subject from the perspective of international relations (IR). In this article, I seek to correct for this theoretical gap and augment the existing understanding of this global development in constitutional design. Using survival analysis, I examine normative, rationalist-materialist, and domestic politics explanations for the phenomenon observed. I find that the adoption of constitutional environmental rights is significantly associated with international civil society influence, human rights legacy, and level of democracy, and best explained by theories of domestic politics and norm socialization. This research suggests that the emergence of constitutional environmental rights signals a major shift in the international normative arena.