While anecdotal evidence suggests that transnational private rule-making organizations (TPROs)—such as eco-certification organizations—lobby public policy makers, we know little about the extent of this phenomenon or the characteristics of TPROs that lobby. TPRO lobbying is relevant given that their rule-making activities directly intersect with public policy. We use the interest group and private governance literatures to examine TPRO features that distinguish TPROs that lobby from those that do not. We developed an original data set of 147 environmental TPROs and assessed TPRO lobbying by their registration in the European Union’s Transparency Register (TR). We find that a quarter of the TPROs in our data set are registered in the TR, and that capacity and expertise matter. Contrary to expectations, however, we do not find that certain key features of TPROs—such as business origins or credibility—are correlated with being registered, which implies that these features do not create inequalities in the TPRO population in terms of lobbying likelihood. By assessing environmental TPROs as interest organizations that engage in lobbying, we contribute to research on public–private governance interactions and identify TPROs as an interest group population in its own right.