This research examines how negative emotions affect the degree to which people endorse conspiracy theories. Conspiracy beliefs typically flourish in contemporary politics that evoke negative emotions. In the present article, we hypothesized that citizens in ressentiment, a particular emotional state salient in contemporary grievance politics, is a key underpinning of why people believe in conspiracy theories. Ressentiment is a multi-layered emotion, within which perceptions of unfairness, bitterness, and feelings of (suppressed) anger are central. Across three studies, we examined the role of ressentiment in predicting belief in conspiracy theories. Study 1 (United Kingdom, N = 300) and Study 2 (United States, N = 300) revealed a positive relationship between ressentiment, measured through a validated scale, and belief in conspiracy theories. This relationship was mediated by rejection of the status quo. In Study 3 (United States, N = 300, pre-registered), we used a vignette describing a fictitious country to experimentally show that evoking ressentiment (vs. control condition) significantly increased belief in conspiracy theories. Taken together, these findings reveal that ressentiment is central for understanding the emotional roots of believing in conspiracy theories.