Nonstate actors play powerful roles in global governance institutions (GGIs) as advocates, experts, representatives, regulators, monitors, and implementing agents. However, the extent to which their inclusion affects the degree to which citizens find GGIs more legitimate has not been systematically investigated, nor have the conditions under which citizens might do so. In this contribution, we theoretically argue that such inclusion effectively enhances the social legitimacy of GGIs, but only to the extent that citizens expect nonstate actors to provide relevant governance contributions. We find strong empirical evidence for this argument in two large-N conjoint experiments fielded in Brazil, Germany, South Africa, and the United States. First, our results suggest that citizens, on average, ascribe more legitimacy to GGIs if learning that nonstate actors have a say in important decisions. Second, the strength of this effect depends on the degree to which citizens expect nonstate actors to provide expertise, representation, public interest orientation, transparency, or operational capacity. Third, expected governance contributions remarkably vary among different types of nonstate actors in kind and degree. In line with our overall argument, findings suggest that nonstate actor participation plays a more complex and significant role in the social legitimation of global governance than previously understood.