So far, the large and expanding body of research on meditation has mostly focussed on the putative benefits of meditation on health and well-being. However, a growing number of reports indicate that psychologically unpleasant experiences can occur in the context of meditation practice. Very little is known about the prevalence and potential causes of these experiences. The aim of this study was to report the prevalence of particularly unpleasant meditation-related experiences in a large international sample of regular meditators, and to explore the association of these experiences with demographic characteristics, meditation practice, repetitive negative thinking, mindfulness, and self-compassion. Using a cross-sectional online survey, 1,232 regular meditators with at least two months of meditation experience (mean age = 44.8 years ± 13.8, 53.6% female) responded to one question about particularly unpleasant meditation-related experiences. A total of 315 participants (25.6%, 95% CI: 23.1 to 28.0) reported having had particularly unpleasant meditation-related experiences, which they thought may have been caused by their meditation practice. Logistic regression models indicated that unpleasant meditation-related experiences were less likely to occur in female participants and religious participants. Participants with higher levels of repetitive negative thinking, those who only engaged in deconstructive types of meditation (e.g., vipassana/insight meditation), and those who had attended a meditation retreat at any point in their life were more likely to report unpleasant meditation-related experiences. The high prevalence of particularly unpleasant meditation-related experiences reported here points to the importance of expanding the scientific conception of meditation beyond that of a (mental) health-promoting and self-regulating technique. We propose that understanding when these experiences are constitutive elements of meditative practice rather than merely negative effects could advance the field and, to that end, we conclude with an overview of methodological and conceptual considerations that could be used to inform future research.