Many international organizations (IOs) rely on voluntary contributions from member states and private actors to fund their operations. Donations from individuals are a significant and increasing income source for these IOs, who rely on marketing strategies such as celebrity endorsement, in the form of Goodwill Ambassadors, to help raise funds. Little is known, however, about the effectiveness of this strategy in the context of IOs although intuition from literatures in marketing and psychology suggests that celebrity endorsement should be effective. We conduct a survey experiment to investigate the effectiveness of Goodwill Ambassadors and, contrary to expectations, find no average effect of celebrity endorsement on donations to, and interest in, IOs and only limited effects among certain sub-groups. We speculate that the context of IOs makes it harder to generate the type of connection between celebrity and cause necessary to make endorsement effective and suggest that further investigation is needed.