How do environmental norms and policies diffuse across borders? In this article, we argue that preferential trade agreements (PTAs) can play an important role in this process. Specifically, we argue that the US has long used PTAs as mechanisms to diffuse such norms, and show this through an empirical examination of three US PTAs, each from a distinct phase of US trade policy. We demonstrate how the US used the North American Free Trade Agreement, the Central American Free Trade Agreement, and the US-Peru Trade Promotion Agreement as vehicles to diffuse norms of (1) public participation in environmental policy-making, and (2) effective enforcement of environmental laws to trading partner nations. In doing so, we both illuminate a new mechanism of environmental norm diffusion and demonstrate the importance of this mechanism in changing environmental policy and practice across borders.