In this article we investigate the institutional mechanisms required for ‘liquid’ forms of authority in transnational governance to achieve normative political legitimacy. We understand authority in sociological terms as the institutionalized inducement of addressees to defer to institutional rules, directives, or knowledge claims. We take authority to be ‘liquid’ when it is characterized by significant institutional dynamism, fostered by its informality, multiplicity, and related structural properties. The article’s central normative claim is that the mechanisms prescribed to legitimize transnational governance institutions – such as accountability or experimentalist mechanisms – should vary with the liquid characteristics of their authority structures. We argue for this claim in two steps. We first outline our theoretical conception of political legitimacy – as a normative standard prescribing legitimizing mechanisms that support authorities’ collectively valuable governance functions – and we explain in theoretical terms why legitimizing mechanisms should vary with differing authority structures. We then present an illustrative case study of the interaction between liquid authority and legitimizing mechanisms of public accountability and pragmatic experimentalism in the context of transnational business regulation. We conclude by considering broader implications of our argument for both the design of legitimate transnational governance institutions, and future research agendas on transnational authority and legitimacy.