The aim of the current scoping review was to explore the associations between religious and spiritual factors and the health-related outcomes of adolescents with chronic illnesses, as well as to investigate possible mechanisms accounting for these relationships. In total, 20 studies meeting the eligibility criteria were reviewed after performing a search of the Scopus, Web of Science, and PubMed databases. The results suggested that religious and spiritual beliefs, thoughts, and practices (e.g., spiritual coping activities) might have both beneficial and deleterious effects on the way adolescents deal with their medical condition, on their psychosocial adjustment, on their mental and physical health, and on their adherence to treatments. Mediating and moderating mechanisms explaining these relations were also evidenced. Suggestions for future research and practical implications for healthcare professionals are provided in the concluding section of this work.