Objective: To provide scientific evidence of the health benefits of forest therapy in terms of physiological indices. Design: Within-group comparison made by conducting field experiments. Participants: Forty-eight young male adults. Methods: Field experiments were conducted at four local sites in Japan. At each site, 12 adults participated in a three-day experiment. To compare physiological reactions between two environmental stimuli, experiments were conducted in forest and urban environments. The participants were randomly assigned to visit either the forest or an urban setting and were instructed to view the landscape in a seated position. The physiological reactions of each participant were recorded before, during, and after viewing the stimuli, and the differences in physiological indices were compared between the two groups. Results: Physiological data revealed that participants demonstrated significantly different reactions in the forest and urban environments. Analysis of heart rate revealed that participants showed a significantly higher ln(HF) and a lower ln(LF/HF) in the forest environment than in the urban environment. Systolic and diastolic blood pressures, pulse rate, and salivary cortisol concentration significantly decreased in the forest environment compared with the urban environment. Subjective evaluation data were generally in accordance with physiological reactions, showing significantly higher scores for “comfortable, natural, soothed, and refreshed feelings” in the forest environment than in the urban environment. Conclusions: This study provided very clear scientific evidence of the physiological effects of forest therapy. Our data indicate that forest therapy can decrease stress and facilitate physiological relaxation.