Multidisciplinary collaboration is key to strengthening the evidence base for multifaceted illness prevention interventions. We bring together health economics and behavioral science to explore the well-being benefits and social cost–benefit of volunteer gardening at an accredited botanic garden, Wales, UK. A health economics-informed social return on investment (SROI) evaluation was combined with the assessment of volunteers’ basic psychological needs (autonomy, competence, and relatedness), connection to nature, and their interrelatedness in this innovative nature-based intervention study. Pre- and post-volunteering outcome data were collected using the Short Warwick-Edinburgh Mental Well-being Scale (SWEMWBS), the ICEpop CAPability measure for Adults (ICECAP-A), the 12-item diary version of the Basic Psychological Need Satisfaction and Frustration Scale (BPNSNF), the Nature Connection Index (NCI), and a bespoke Client Service Receipt Inventory (CSRI). Results indicate that volunteer gardening can provide well-being benefits to participants and cost savings to the NHS. The well-being benefits observed were estimated to generate social value in the range of GBP 4.02 to GBP 5.43 for every GBP 1 invested. This study contributes to the evidence base that simple nature-based interventions such as volunteer gardening could offer low-cost supportive environments that deliver significant well-being benefits and associated social value to local communities, including a reduced burden on overstretched local healthcare services.