I argue that transnational law has a formal legal character. To do so, I build on socio-legal scholarship in this domain, but apply a jurisprudential perspective. Specifically, I argue that public fiduciary theory is well-placed to explain the nature of authority evoked by transnational legal orders, as well as their legal features and constitution. This theory claims that the fiduciary character of an organization - that is, its other-regarding purpose and adherence to certain procedural standards - is a necessary feature of its claim to legitimate authority. By addressing these jurisprudential questions, public fiduciary theory contests scholars who deny transnational law’s formal legal character. Ultimately, I argue that despite the private constitution of some transnational lawmaking bodies, and their lack of express public authorization, by fulfilling a transnational fiduciary role they exercise authority that either is or closely resembles public authority. This, in turn, contributes to the legal character of the transnational norms these bodies generate. I use the International Organization for Standardization as an illustrative case study. The implications of this approach include acknowledging that law can exist beyond national or international state regulation, and a denial that coercion is an essential element of law.