Objective: To investigate the effect of diabetes self-management education and support via a smartphone app in individuals with type 2 diabetes on insulin therapy. Research design and methods: Open two-arm multicenter parallel randomized controlled superiority trial. The intervention group (n=115) received theory and evidence-based self-management education and support via a smartphone app (optionally two or six times per week, once daily at different times). The control group (n=115) received care as usual. Primary outcome: HbA1c at 6 months. Other outcomes included HbA1c ≤53 mmol/mol (≤7%) without any hypoglycemic event, body mass index, glycemic variability, dietary habits and quality of life. We performed multiple imputation and regression models adjusted for baseline value, age, sex, diabetes duration and insulin dose. Results: Sixty-six general practices and five hospital outpatient clinics recruited 230 participants. Baseline HbA1c was comparable between groups (8.1% and 8.3%, respectively). At 6 months, the HbA1c was 63.8 mmol/mol (8.0%) in the intervention vs 66.2 mmol/mol (8.2%) in the control group; adjusted difference -0.93 mmol/mol (-0.08%), 95% CI -4.02 to 2.17 mmol/mol (-0.37% to 0.20%), p=0.557. The odds for achieving an HbA1c level ≤7% without any hypoglycemic event was lower in the intervention group: OR 0.87, 95% CI 0.33 to 2.35. There was no effect on secondary outcomes. No adverse events were reported. Conclusions: This smartphone app providing diabetes self-management education and support had small and clinically not relevant effects. Apps should be more personalized and target individuals who think the app will be useful for them.