Objective To investigate the effects of an exercise behavior intervention based on the health action process approach (HAPA) model on exercise adherence, self-efficacy, and glycemic control in patients with type 2 diabetes mellitus (T2DM). Methods A retrospective cohort of 76 T2DM patients managed at Chuzhou Integrated Traditional Chinese and Western Medicine Hospital from January 2023 to October 2025 was selected. Participants were divided into a control group (n＝38, receiving standard diabetes management) and an intervention group (n＝38, receiving standard diabetes care combined with HAPA-based exercise behavior intervention). Changes in exercise adherence, exercise self-efficacy, and glycemic indicators [fasting plasma glucose (FPG), glycated hemoglobin (HbA1c)] were compared before and after intervention. Results Post-intervention, the intervention group demonstrated significantly greater improvements than the control group in exercise frequency, duration, behavioral persistence, and self-monitoring dimensions (all P＜0.05). Improvements in exercise self-efficacy (situational, obstacle-overcoming, and maintenance efficacy) also exceeded those of the control group (all P＜0.05). Regarding glycemic control, the intervention group demonstrated significantly greater reductions in FPG and HbA1c compared to the control group (both P＜0.05). Conclusion The HAPA-based exercise behavior intervention effectively enhances exercise adherence and exercise self-efficacy in patients with T2DM, further improving glycemic control.