Private rule-setting organizations increasingly design, implement, and monitor rules and standards that prescribe behavior in the global governance for sustainability. In this article we develop criteria against which we evaluate the output legitimacy of these organizations along two dimensions on the basis of their acceptance by different constituencies. The internal dimension refers to the acceptance of the organization's rules and standards by the relevant target group, and is assessed on the basis of standard uptake and compliance. The external dimension signifies the ability of the organization to have broader political and socio-economic impact that reaches beyond the target group, and is evaluated on the basis of structural, cognitive, and regulatory effects. With reference to the Marine Stewardship Council and Friend of the Sea, our analysis illustrates that while claims by private organizations to output legitimacy are not unfounded in sustainability governance, they can also be contested when considered in a global context.