Background <p>Anxiety, depression and stress-related conditions are prevalent and growing public health challenges, impacting individuals’ quality of life and placing significant strain on healthcare systems globally. These complex conditions rarely respond to a single mode of treatment, underscoring the importance of diverse and complementary approaches. Nature-based health interventions (NBHIs) represent one such perspective, offering novel opportunities to address mental health by engaging with natural environments. While international interest in NBHIs is growing, evidence on how these interventions can be systematically developed and implemented remains limited. The aim of this study protocol is to report the planned Nature Impact Feasibility Study.</p> Methods <p>The Nature Impact Feasibility Study is a one-armed, mixed methods feasibility study aiming to evaluate a complex, group-based NBHI developed using the Medical Research Council framework for complex interventions. The intervention will be conducted across three sites in Denmark: Glostrup outpatient clinic, Kolding and Silkeborg municipality centres. The intervention consists of group-based NBHI programmes delivered weekly over 10 consecutive weeks, facilitated by experienced health professionals. The feasibility evaluation will examine three core domains. These include (1) the implementation process, including fidelity, dose, reach, adaptations and acceptability; (2) mechanisms of change and (3) contextual factors influencing implementation and intervention processes. Data will be collected through participant observations, focus group interviews, registration forms and participant questionnaires assessing mental health outcomes and the participants’ relationship to nature. Data will be analysed using a mixed methods convergent design. Quantitative and qualitative data will be analysed separately and then integrated thematically during interpretation.</p> Discussion <p>By reporting the study protocol for the Nature Impact Feasibility Study in advance, the project enhances transparency, allows for critical appraisal of the planned design and methods and strengthens the foundation for the subsequent feasibility evaluation and future large-scale trials. The Nature Impact Feasibility Study will provide valuable insight into delivering NBHIs for adults experiencing mild to moderate anxiety, depression and/or stress, while informing intervention refinement, outcome measures and future research aimed at investigating the potential of integrating NBHIs into existing practices for treating anxiety, depression and stress.</p> Trial registration <p>NCT06981000. Registered on March 13, 2025.</p>