Objectives: To clarify the physiological and psychological effects of a one-day forest therapy. Methods: The experiments were conducted at three sites, each containing a forest area and an urban area in Japan. Twelve male students participated at each of the three venues (total, 36 subjects). The subjects were randomly assigned to visit either the forest or urban area and were instructed to view the scenery in a seated position for 15 min. Heart rate variability and heart rate were measured to assess physiological response. The semantic differential method, reports of “refreshed” feeling, “state anxiety” in the State-Trait Anxiety Inventory (STAI) and Profile of Mood States (POMS) were used to determine psychological responses. Results: Physiological data revealed that the subjects demonstrated significantly different responses to the forest and urban environments. For heart rate variability, the subjects showed a significantly higher ln(HF) and a lower ln(LF/HF) in the forest environment than in the urban environment. Heart rate decreased in the forest environment relative to that in the urban environment. The forest environment was perceived as significantly more “comfortable,” “soothing,” and “natural” than the urban environment after viewing. The score for “refreshed” feeling was also significantly higher. The score for the “state anxiety” in STAI was significantly reduced by viewing the scenery in the forested areas. The score for the “vigor” subscale of the POMS was significantly improved by viewing the scenery in the forested areas, whereas scores for negative feelings, such as “tension–anxiety” and “fatigue,” were significantly reduced. Conclusions: These results provided scientific evidence of the physiological and psychological effects of a one-day forest therapy.