Social Prescribing (SP) involves linking individuals with mental illness to local health and welfare services to improve quality of life (QoL) and biopsychosocial wellbeing. SP programs address psychosocial wellbeing by linking individuals to group activities. Forest Therapy (FT) is a group nature walk with prescribed activities that promote mindfulness, relaxation, and shared experience. Improvements in psychological and physical wellbeing have been demonstrated in FT, but psychosocial impacts have not been widely investigated. This study will implement an SP FT intervention and assess the impacts on QoL and biopsychosocial wellbeing. Participants will include 140 community-living adults with mental illness at Sydney/Gold Coast, Australia. A stepped-wedge cluster randomised design will be used; each participant will complete a 10-week control period followed by a 10-week FT intervention. Weekly 90-min FT sessions will be conducted in groups of 6–10 in local nature reserves. Validated tools will measure self-report QoL and biopsychosocial wellbeing pre-and post-control and intervention periods, and 5-week follow-up. Blood pressure and heart rate will be measured pre-and post-FT sessions. Hypothesised outcomes include improvements in QoL and biopsychosocial wellbeing. This study is the first to assess SP FT, and may provide evidence for a novel, scalable mental illness intervention.