Introduction: Despite extensive technical transparency and international oversight, Japan's release of ALPS-treated water from the Fukushima Daiichi nuclear plant continues to face domestic and international opposition. This persistent skepticism highlights the limits of procedural fairness alone in fostering public acceptance of controversial policies, particularly in high-risk and emotionally charged contexts. Trust in scientific evidence and institutional transparency, while essential, may be insufficient when cognitive biases and perceptions of injustice are at play. Objective:bThis study examines the psychological mechanisms underlying public (un)support for crisis policies. Specifically, it investigates the role of Zero-sum Thinking—the belief that government actions disproportionately benefit elites at the expense of ordinary citizens—and procedural justice in shaping policy legitimacy. By integrating these perspectives, the study seeks to illuminate why resistance endures even when procedural safeguards are upheld. Method: A cross-sectional survey was conducted using a stratified sample of Japanese residents, balanced by age and gender. Respondents evaluated procedural dimensions (e.g., transparency, participatory voice), risk perceptions, and cognitive biases in relation to policy acceptance. Conditional process analysis was employed to test mediation and moderation pathways, clarifying how procedural fairness interacts with zero-sum beliefs and risk perception to influence public support. Discussion: The findings aim to advance theoretical models of crisis communication by incorporating cognitive bias into frameworks of legitimacy and trust. This approach extends beyond institutional integrity to explain why policies backed by scientific consensus still encounter resistance. By demonstrating the interplay between fairness perceptions and zero-sum thinking, the study contributes a new lens for understanding public skepticism toward scientifically sound but socially contested policies. Ultimately, these insights may inform more effective strategies for communicating risk and fostering legitimacy in contexts of crisis governance.