Background: Psilocybin-assisted therapies (PAT) are reemerging as a treatment for complex distress often prompting mystical experiences, enhanced meaning, and spiritual wellbeing. We sought to investigate how measures of spirituality are employed in experimental studies of PAT conducted with seriously ill adults. Methods: We included experimental studies of psilocybin conducted with seriously ill adults, which employed measures that contained spirituality and mysticism concepts within their domains or subdomains. Included studies were peer-reviewed and published in English language (up to December 2021). Results: Seven articles met our inclusion criteria. A total of 12 unique instruments were identified. The most frequently used instruments were the Mystical Experience Questionnaire (MEQ30), the Functional Assessment of Chronic Illness Therapy-Spirituality (FACIT-Sp-12), and the Demoralization Scale (DS-I/II) (used in four studies each), followed by the Persisting Effects Questionnaire (PEQ) (used in three studies). Overall, studies did not consistently define and contextualize spirituality domains and subdomains studied. Conclusions: Despite well-recognized significance of spirituality in PAT, there was considerable heterogeneity in number and types of spirituality measures employed across studies. There also seemed a lack of attention to defining and operationalizing spirituality and its domains and subdomains. This is notable as spirituality and overlapping concepts (eg mystical experiences) contributes substantially to this body of research and patients’ therapeutic outcomes. Towards developing more rigorous science of spirituality in PAT research, there is a critical need to evaluate and refine measures of spirituality to enhance their utility and replicability, limit participant burden, and better contextualize spirituality-related findings and outcomes.