AbstractResearchers have theorized about how belief in conspiracy theories can negatively affect interpersonal relationships. However, despite growing anecdotal evidence of the effects that conspiracy theories seem to have on people's relationships, a systematic assessment of these effects is lacking. In seven studies (six of them preregistered, N = 2526), we examined whether people's perceptions of others' conspiracy beliefs were negatively associated with their actual (or anticipated) relationship satisfaction with those others. We found that participants' perceptions of their social contacts' beliefs in general (Pilot Studies 1–2) and specific conspiracy theories (Study 1) were negatively associated with their relationship satisfaction with those contacts. Using a hypothetical scenario, we further observed that participants anticipated that their relationship satisfaction would worsen when one of these social contacts explicitly endorsed (vs. opposed) a conspiracy theory (Studies 2, 3a, and 3b). Finally, participants expected lower relationship satisfaction with a stranger who endorsed (vs. opposed) a conspiracy theory in their online dating profile (Study 4). Importantly, across all studies we observed that participants' own conspiracy beliefs moderated the association between others' conspiracy beliefs and relationship satisfaction, revealing a similarity–dissimilarity pattern: although the association was negative among participants with weaker conspiracy beliefs, we observed signals of reversal among participants with stronger conspiracy beliefs. Our findings further suggest that a process of attitudinal distancing (among other relational changes) could explain why perceived conspiracy beliefs negatively predicted relationship satisfaction. Taken together, this research provides evidence that conspiracy beliefs have the potential to harm interpersonal relationships.