Background: This study investigated the effectiveness of a social networking site (SNS)-based automatic mobile message providing system on glycemic control in patients with type 2 diabetes mellitus (T2DM). Methods: A 3-month, randomized, open-label, controlled, parallel-group trial was conducted. One hundred and ten participants with T2DM were randomized to a mobile message system (MMS) (n=55) or control group (n=55). The MMS group received protocolbased automated messages two times per day for 10 weeks regarding diabetes self-management through KakaoTalk SNS messenger. The primary outcome was the difference in the change in glycated hemoglobin (HbA1c) levels (%) from baseline to week 12. Results: HbA1c levels were more markedly decreased in the MMS group (8.4%±0.7% to 8.0%±1.1%) than in the control group (8.5%±0.8% to 8.4%±0.8%), resulting in a significant between-group difference (P=0.027). No differences were observed in changes in fasting glucose levels, lipid profiles, and the number of participants who experienced hypoglycemia, or in changes in lifestyle behavior between groups. However, the self-monitoring of blood glucose frequency was significantly increased in the MMS group compared to the control group (P=0.003). In addition, sleep duration was increased in the MMS group, but was not changed in the control group. Conclusion An SNS-based automatic mobile message providing system was effective in improving glycemic control in patients in T2DM. Studies which based on a more individualized protocol, and investigate longer beneficial effect and sustainability will be required in the future.