<p>Mystical experiences are among the most extraordinary and meaningful experiences one can have. While such experiences have been studied for more than a century, it is still an open question how the brain can produce them. The present paper provides a possible answer by examining mystical experience from the lens of the REBUS hypothesis—a theory that explains the neurocognitive mechanisms of psychedelics in the Bayesian brain. Since psychedelics have consistently been shown to induce mystical experiences, the REBUS hypothesis implicitly suggests that it can also shed light on the underlying mechanisms of mystical experiences. The resulting account holds that during a mystical experience, the brain’s constant process of prediction error minimization is characterized by a decreased influence of high-level predictions and, simultaneously, an increased influence of sensory input. The latter can be related to the noetic quality of mystical experiences; the former to the experience of oneness, altered perception of time and space, ineffability, and deeply felt positive mood during/after a mystical experience. While the paper’s account is developed on a conceptual basis and thus remains speculative, it is compatible with other models of mystical experience, such as that mystical experience involves maximal uncertainty or a breaking down of attention.</p>