During COVID-19, conspiracy theories were intensely discussed in the media. Believing in specific conspiracy theories (ie, explanations for events based on powerholders’ secret arrangements) as well as the general tendency to believe in conspiracy theories—a so-called conspiracy mentality—have been found to predict cognition and behavior with negative societal effects, such as low institutional trust. Accordingly, believing in conspiracy theories around COVID-19 should work against institutional trust, support of governmental regulations and their adoption, as well as societal engagement (eg, helping members of risk groups). We tested these predictions in a national random sample, an experimental study, and a longitudinal study (Ntotal= 1,213; all studies preregistered). Indeed, believing in and being confronted with a COVID-19 conspiracy theory decreased institutional trust, support of governmental regulations, adoption of physical distancing, and—to some extent—social engagement. Findings underscore that conspiracy theories have severe societal effects in the context of COVID-19.(150 words)