Forests provide important ecosystem services that extend beyond material resources to include significant benefits for human mental health and well-being. A growing body of interdisciplinary research indicates that forest exposure can produce short-term reductions in stress, anxiety and negative affect, while repeated or structured interventions may support broader improvements in mood, attention and psychological well-being. This review synthesises current evidence on the mental health benefits of forest visits, with a particular focus on forest ecosystems, and examines the underlying mechanistic pathways linking forest exposure to psychological outcomes. These mechanisms include stress reduction and attention restoration, multisensory environmental effects, exposure to natural bioactive compounds, promotion of physical activity and social interaction, and immune modulation via microbial diversity. In parallel, the review evaluates emerging evidence on digital forest therapies, such as virtual reality, 360° forest videos and mobile applications, which aim to deliver nature-based benefits to individuals with limited access to real forests. While digital approaches can provide short-term improvements in mood and stress recovery, current evidence remains heterogeneous, and these interventions generally lack the full sensory, biological and behavioural complexity of real forest environments. We discuss the strengths and limitations of both real and digital forest exposure and highlight their complementary roles. Finally, we consider implications for public health, forest management and policy, emphasising the need to recognise mental health benefits as a core ecosystem service and to integrate forest-based interventions into preventative health strategies.