<p>This study examines the signaling effect of support from nonprofit experts on celebrity credibility regarding advocacy efforts. Furthermore, we provide detailed insights into celebrity–nonprofit partnerships by exploring the congruence between the celebrity and advocacy content, and examining whether the impact of support from nonprofit experts differs based on the celebrities’ source characteristics. A survey experiment manipulating the support of nonprofit organizations for celebrity advocacy was conducted, and data from 776 Japanese respondents collected through an online panel were analyzed. The results indicated that signals of nonprofits’ support significantly enhanced perceptions of celebrity expertise but only when strong congruence existed between the celebrity and advocacy content. However, these signals did not enhance the perception of celebrity trustworthiness. We present a causal explanation of how nonprofit experts’ support affects perceived celebrity expertise, which ultimately leads to increased cognitive involvement in social issues. The findings integrate signaling theory and the source credibility model to explain the mechanisms underlying celebrity–nonprofit partnerships in advocacy efforts.</p>