Both exposure to nature and physical exercise have been shown to have positive effects on mental well-being. We reviewed the combined effects of physical exercise in nature (i.e., ‘green exercise’) on mental well-being. A systematic review of the databases Ovid Medline, PubMed and PsycINFO resulted in a total of 57 included studies (of which 25were meta-analysed). All eligible studies compared a green exercise intervention with: (1) a no intervention control group, (2) indoor exercise, (3) urban exercise, or (4) other interventions to improve mental well-being. Studies without a comparison group were excluded. Our results show that green exercise interventions have a positive effects on mental well-being (0.478; p = 0.001; 95% CI = [0.191, 0.766]). Subgroup analyses revealed that green exercise interventions had more positive effects on mental well-being compared to no-intervention control groups (5 studies; 0.851, se = 0.248, p < 0.001) and other mental well-being interventions (8 studies; 0.540, se = 0.188, p = 0.05), but not compared to indoor (5 studies; 0.04, se = 0.203, p = 0.819), or urban exercises (10 studies; 0.415, se = 0.268, p = 0.124). While green exercise clearly outperforms no activity and non-physical interventions in enhancing mental well-being, its benefits over other forms of physical activity may be more nuanced, potentially moderated by factors such as duration, environmental quality, and measurement sensitivity. Future interventions should explore what types of green exercise are the most beneficial, and which populations may benefit the most from participation in green exercise (e.g., clinical, youth, migrant communities).
