AbstractThe Japanese government's decision to discharge treated water into the ocean has drawn citizens' attention, and the evaluation of this issue remains controversial. The themes underlying social issues are connected to the moral foundation of Haidt's moral foundations theory. Accordingly, these foundations appeal to individuals' subjective approval of their moral foundations. The treated water issue is related to the purity foundation; thus, the present study aimed to investigate the role of the purity foundation on the support for the treated water issue by collecting data from 1,291 Japanese participants immediately after the first discharge of treated water. To measure the purity‐related concept, the Purity Orientation–Pollution Avoidance Scale (POPA) was employed. The results of our multiple regression analysis showed a negative association for the Mental Purity subscale and a positive association for the Respect for Religion subscale of POPA. The different aspects of Japanese purity highlighted by POPA are discussed. This study contributes to a deeper understanding of the mechanisms involved in people's attitudes toward social issues.