Purpose: This study aimed to identify the effects of the Horticultural Therapy Program on interpersonal relations, self-efficacy, and stress awareness for patients with mental illness.Methods: This was a non-equivalent control group pre and post-test design study. Participants were recruited from a closed ward of mental health hospitals located in the G city. 23 patients from two hospitals were in the experimental group and 20 patients from four hospitals were in the control group. A ninety-minute horticultural program was performed once a week for 8 weeks.Results: Results revealed that the Horticultural Therapy Program was effective for self-efficacy (z=-3.06 <i>p</i>=.002) and stress awareness (z=2.92, <i>p</i>=.004) for patients with mental illnesses.Conclusion: These results indicate that the Horticultural Therapy Program is effective for self-efficacy and stress awareness in patients with mental illnesses. Therefore, the Horticultural Therapy Program is recommended as an effective psychiatric nursing intervention for mental health promotion and rehabilitation.