There has been an increasing interest in the relationship between religion and psychosocial functioning during the COVID-19 pandemic. Interestingly, emerging recent findings suggest that religiousness may have a Janus-face impact on how people cope with the pandemic, leading to both positive and negative social outcomes. In this project, we examine whether two types of religiousness (i.e., centrality of religiosity and religious fundamentalism) are associated with COVID-19 conspiracy beliefs and socially undesirable behavior during the pandemic. We suggest that only the most dogmatic and fundamentalistic type of religiousness could lead to conspiracy beliefs, while centrality of religiosity could be unrelated or even negatively related to this type of thinking. In a series of two studies (N = 361 and N = 394) conducted among Polish Roman Catholics, we demonstrate that religious fundamentalism, unlike centrality of religiosity, is positively related to coronavirus conspiracy beliefs, which, in turn, promote socially maladaptive behavior such as freeriding or non-adherence to safety guidelines. •Religious fundamentalism (RF) is positively related to COVID-19 conspiracy beliefs.•Centrality of religiosity is either unrelated or negatively related to COVID-19 conspiracy beliefs.•COVID-19 conspiracy beliefs mediate the link between RF and maladaptive behavior.