Objective An increase in psychological flexibility has been found to be associated with health behavior changes. Peer-led interventions have been advantageous in improving physical activity among individuals at health risk. This study aimed to discover whether an ACT-based peer-tutored online intervention can increase self-reported physical activity participation and psychological flexibility among adults with overweight. Design The study was a non-randomized longitudinal intervention study. The intervention participants (N = 177) were primary health care clients with overweight. They participated in a 24-month program provided by health services, including three online modules of ACT of six week each, and tutoring by trained peers via five group meetings and four phone calls. Main outcome measures and results Physical activity participation was measured with Kasari’s FIT index concerning the frequency, intensity and time of the physical activity. Psychological flexibility was measured with AAQ-II, and thought suppression, as a dimension of psychological flexibility, with WBSI. Measures were taken at baseline, and at 6, 12, and 24 months. The statistical analysis was conducted with Mplus to identify latent groups with similar change patterns of physical activity, and to examine differences between the profiles. Two change profiles for physical activity participation were found: Low and High. At baseline in High profile group, physical activity and psychological flexibility were higher and thought suppression was lower than they were in Low profile group, as was expressing other psychological symptoms measured by DASS. During the intervention, physical activity increased significantly only within Low profile (within Cohen’s d = .48). Psychological flexibility (AAQ-II) increased within High profile (within Cohen’s d = .34), and thought suppression (WBSI) decreased in both profiles (within Cohen’s d = .33). Conclusion The ACT-based peer-tutored online intervention was promising especially for participants with low physical activity participation.