Introduction: Smartphone-based education has been proven to indirectly improve clinical outcomes of type-2 diabetes mellitus patients via practical disease management. “Teman Diabetes” is a new smartphone application that helps diabetics better manage their diabetes, and allows collaboration between diabetics, their inner circle, and medical professionals. However, there is still limited evidence about the effectiveness of “Teman Diabetes” on clinical outcomes of diabetic patients, especially type-2 diabetes mellitus. The objective of this research is to determine the positive effect of smartphone application-based education “Teman Diabetes” on the clinical outcomes of type-2 diabetes mellitus patients. The clinical outcomes measured were HbA1c and fasting plasma glucose levels. Method: Using a quasi-experimental design, a total of 83 participants were selected by consecutive sampling. Patients in the intervention group (n = 44) received "Teman Diabetes" when they assigned the informed consent form. Furthermore, they received a WhatsApp message containing a different link from the "Artikel" feature on the Teman Diabetes every week from the research team, from the second week to twelfth week. The control group (n = 39) only received standard therapy without any additional interventions. Patients in both groups were followed before and after 3 months (HbA1c and fasting plasma glucose levels). The statistical test used in this study is Mann-Whitney test for hypotheses and comparison of mean changes between groups; Wilcoxon test for comparison of mean changes between treatments; and analysis of covariance (ANCOVA) test for the effect of confounding variables. Results: After 3 months, the mean HbA1c reduction was greater in the intervention group by -0.7 ± 0.9% (P<0.001) than in the control group by -0.1 ± 1.1% (P=0.17). Also, the mean fasting plasma glucose reduction was greater in the intervention group by -25.0 ± 71.4mg/dl (P=.02) instead of in the control group by 7.6 ± 72.4mg/dl (P=0.78). It is concluded that smartphone application-based education “Teman Diabetes” give a significant positive effect on clinical outcomes among type-2 diabetes mellitus patients. The main features that contributed to the clinical outcome of this study were "Artikel", "Forum", and "Edukasi" feature.