Drawing from social interdependence theory, self-categorization theory (SCT), and normative social influence, this study explores how nonprofit organizations can harness celebrity fandoms to drive the influence of celebrity endorsements. A survey was conducted based on the collaboration between the famous South Korean band BTS and the United Nations (UN) in sustainability communication. The results emphasize a link between fans' perceptions of participatory fandom culture and their willingness to engage in advocacy behaviors related to issues championed by celebrities. These perceptions are influenced by social interdependence among fans and the celebrity, prevailing fandom norms, and individual identification with the fandom. Nonprofit organizations can amplify advocacy efforts by strategically partnering with celebrities with interdependent and participatory fan bases. Customizing communication campaigns to fan communities' interests and norms can enhance effectiveness. This study also provides insights for fandom managers aiming to build robust fan communities. The study contributes to the discourse on public relations and the impact of passionate publics on nonprofit communications. It marks one of the initial attempts to quantify the influence of participatory fandom culture.