Despite a growing interest in the relationship between religion and spirituality (RS) and mental health across helping professions, less is known about clients’ perceived relevance of these areas. This article describes the development and validation of the Relevance of Religion and Spirituality to Mental Health (RRSMH) scale, and responses to the first national survey of clients’ perceived relevance of RS to mental health. Specifically, a sample of 989 U.S. adults who saw a mental health care provider in the last month responded to an online survey that included 27 new items to measure clients’ perceptions of the relevance of RS to mental health, both positive and negative. A confirmatory factor analysis revealed that the sample’s data had an adequate fit to the final 12-item model, and the instrument’s overall reliability was very good (α = .96). Descriptive analyses indicated that clients view RS as both supportive and relevant to their mental health. The RRSMH scale may be used in mental health research and practice settings. Authors recommend that RS be assessed and included in treatment planning, where appropriate, and addressed in training for mental health professionals.