Objectives <p>This study aimed to evaluate the effects of an 8-week mindfulness intervention integrated into a weight management program comprising education and exercise components for young adults with overweight or obesity.</p> Method <p>Participants (mean age, 20.00 ± 1.12&#xa0;years; mean body mass index, 30.53 ± 6.37&#xa0;kg/m²) were randomly assigned to a control group, a core stabilization group, or a mindfulness plus core stabilization group. Health behavior outcomes, including unhealthy dietary patterns, exercise adherence, physical activity enjoyment, sedentary behavior, sleep quality, mental health symptoms, and satisfaction with health behavior, were assessed alongside physical fitness measures at baseline, 4&#xa0;weeks, and 8&#xa0;weeks.</p> Results <p>The results demonstrated that integrating mindfulness significantly improved exercise adherence (<i>p</i> = 0.002; 24.43% increase; <i>ω</i>² = 0.076); enhanced coping-related mental health as evidenced by reductions in mental health symptoms, particularly stress (<i>p</i> = 0.002; 25.23% decrease; <i>ω</i>² = 0.084); and increased participants’ self-reported satisfaction with their health behavior (<i>p</i> &lt; 0.001; 35.45% increase; <i>ω</i>² = 0.276). Furthermore, integrating mindfulness practices into weight management programs may promote sustained improvements in body composition. All groups exhibited enhancements in cardiovascular fitness, muscle strength, and balance.</p> Conclusions <p>Mindfulness augmented the benefits of conventional education and exercise interventions for improving health behavior and managing body composition in young adults who were overweight or obesity. The findings of this study support mindfulness-based mental training as an effective strategy for fostering healthier behavior in young adults.</p> Preregistration <p>Thaiclinicaltrials.org, TCTR20250609001, 9 June 2025; Open Science Framework, osf.io/saj8k, 25 March 2026. Registrations were retrospective.</p>