Overconfidence have been argued to be associated with conspiracy beliefs, yet their relationships have not been clearly tested. In this research, we examined both correlationship and causal relationship. In Study 1 (N = 344), we explored the correlationship between overconfidence and conspiracy beliefs. We found that both overconfidence in general knowledge and overconfidence in climate change knowledge were positively correlated with climate change conspiracy beliefs. In Study 2 (N = 299), we tried to figure out the role of overconfidence in a setting where people were exposed to a conspiracy theory (death of Epstein). We found that people are more susceptible to conspiracy theories to the extent they are more overconfident. In Study 3 (N = 329), we measured conspiracy theories about COVID-19 and overconfidence in knowledge of COVID-19. Consistent with Study 1, overconfidence was positively correlated to conspiracy beliefs. Conspiracy beliefs did not facilitate overconfidence.