Although legitimacy is crucial for courts’ efficacy, the sources identified as legitimizing domestic institutions are weaker or absent altogether for international institutions. We use an original, preregistered, nationally representative survey experiment to show that perceived home-state interest strongly affects the legitimacy afforded by UK citizens to the International Criminal Court. Importantly, this relationship is moderated by nationalism. Our findings have implications for state actors in a position to act vis-á-vis international courts, elites seeking to alter opinions toward courts, and courts themselves weighing possible institutional costs of acting against noncompliant states.