Cognitive and evolutionary research has overwhelmingly focused on the powerful deities of large-scale societies, yet little work has examined the smaller gods of animist traditions. Here, in a study of the Mentawai water spirit Sikameinan (Siberut Island, Indonesia), we address three questions: (1) Are smaller gods believed to enforce cooperation, especially compared to bigger gods in larger-scale societies? (2) Do beliefs in these deities encourage people to engage in behavior that would otherwise be perceived as costly? and (3) Does ritual reinforce beliefs in these deities? Drawing on interview responses, data from healing ceremonies, and ethnographic observation, we show that Sikameinan is believed to punish people who violate meat-sharing norms and that people ‘attacked’ by Sikameinan pay shamans to conduct healing rituals. The public nature of rituals, involving prestigious individuals apologizing to Sikameinan for the patient’s stinginess, reinforce onlookers’ beliefs about Sikameinan. The most widely shared beliefs about Sikameinan are represented in rituals while beliefs not represented vary considerably, indicating that ritual may be potent for cultural transmission. These results suggest that moralizing supernatural punishers may be more common than suspected and that the trend in the cultural evolution of religion has been an expansion of deities’ scope, powers, and monitoring abilities.