--- - |2 Abstract Conspiracy theories (CTs) about government officials and the institutions they represent are widespread, and span the ideological spectrum. In this study, we test hypotheses suggesting that system identity threat, or a perception that society's fundamental, defining values are under siege due to social change, will predict conspiracy thinking. Across two samples (N = 870, N = 2,702), we found that system identity threat is a strong predictor of a general tendency toward conspiracy thinking and endorsement of both ideological and non‐ideological CTs, even after accounting for numerous covariates. We also found that the relationship between system‐identity threat and conspiracy‐theory endorsement is mediated by conspiracy thinking. These results suggest that conspiracy‐theory endorsement may be a compensatory reaction to perceptions that society's essential character is changing. - European Journal of Social Psychology, EarlyView.