This quasi-experimental study aimed to investigate the effects of gymnastics-based nursing intervention on stress, obesity, and mental health confidence in patients with chronic mental illness. Methods: The study utilized the non-equivalent control group pretest-posttest design and recruited sixty patients with chronic mental illness who were registered at one of the two mental rehabilitation centers in D metropolitan city. Data were collected from May 2017 to August 2017. The experimental intervention was a gymnastics-based program developed by the main investigator based on Bandura's self-efficacy theory. The program was administered twice a week for a total of 16 sessions. Results: This study showed that subjective stress responses (z=-3.47, p=.001), physiological stress responses (z=-2.90, p=.004), body weight (F=3.55, p=.032), abdominal circumference (t=-2.19, p=.032), and optimism in the mental health confidence domain (t=2.27, p=.027) were significantly different in the experimental group and control group. But, results showed that body fat percentage (F=0.91, p=.406) was not significantly different in the experimental group and control group. Conclusion: The gymnastics-based nursing intervention was effective in reducing stress, body weight, and abdominal circumference, and improving optimism in the mental health confidence domain in patients with chronic mental illness.