Interdisciplinary approaches are widely advocated in global science policy as a key strategy for addressing complex social issues. However, evidence linking interdisciplinary research to its impact on the public remains limited. It is generally believed that interdisciplinary research produces solutions to critical problems by recombining knowledge from multiple fields. Yet the public often struggles to appreciate this recombination, since most individuals are not as specialized as patent examiners. To address this gap, our study introduces a more accessible attribute of interdisciplinary scientific knowledge—that of being a public good—alongside its re-combinatory nature, to explain how interdisciplinary citizen science fosters public engagement. Drawing on classic signaling theory from product management research, as well as social cognition theory and science communication theories, we focus on the signaling system formed by citizen science project teams, the public, and society as a whole. Based on this framework, we develop a theoretical model to examine how interdisciplinarity influences the public’s willingness to participate in citizen science projects. According to social role theory, we also consider how a research team’s identity might moderate the effect of interdisciplinarity on engagement willingness. Our study analyzes 880 citizen science projects hosted on Experiment (experiment.com) and engagement data from 37,867 contributors. We employ text‐mining methods—including Latent Dirichlet Allocation topic modeling and Word2Vec—to quantify three dimensions of interdisciplinarity in project titles and descriptions: diversity, balance, and disparity. Validation through manual checks and algorithmic tests confirms the robustness of our quantification approach, demonstrating that text mining effectively identifies disciplinary topics. Empirical results show that interdisciplinarity in citizen science projects promotes public engagement through two mechanisms. First, there is a direct positive effect: projects spanning diverse disciplines attract both broader and deeper public engagement. Second, there is an indirect positive effect: interdisciplinarity reduces a project’s novelty, thereby mitigating novelty’s negative impact on public engagement willingness; this indirect effect, however, pertains only to public engagement breadth. We find that when the research team is affiliated with academia, the direct positive effect of interdisciplinarity on public engagement breadth is strengthened, although no significant moderation appears for public engagement depth. Furthermore, when disciplinary diversity or balance is high, teams composed of both academic and amateur members negatively moderate interdisciplinarity’s impact on public engagement breadth. Under conditions of high disciplinary balance, such mixed teams also negatively moderate the effect on public engagement depth. In contrast, amateur teams’ identity shows no significant moderating effects. Using the Web of Science classification system and project tags, we classify citizen science projects as either natural science-oriented or humanities and social science-oriented. In natural science-oriented projects, interdisciplinarity enhances both public engagement breadth and depth, and the moderating effects of team identity align with our main findings. However, in humanities and social science-oriented projects, interdisciplinarity negatively affects public engagement breadth, though academic team can offset or even reverse this negative effect. This study enriches understanding of how interdisciplinary research influences public engagement. Practically, it offers guidance for government agencies and scientists in managing interdisciplinary citizen science activities.