Introduction: Psychological distress was experienced by millions of people affected by the coronavirus (COVID-19) pandemic, particularly adolescents with diabetes. Method: A cross-sectional study was conducted among adolescents with type 1 (T1DM) and type 2 diabetes mellitus (T2DM) aged 10-18 years during the nationwide lockdown from June-December 2020 in Malaysia to assess the impact of COVID-19 towards their mental health. Eighty-seven participants were recruited and interviewed regarding their lifestyle changes, followed by the administration of the Depression, Anxiety and Stress Scale (DASS-21). Results: Fifty-eight patients with T1DM (30 males, 51.7%) and 29 patients with T2DM (9 males, 31%) participated in the study. The incidence of depressive, anxiety and stress symptoms were detected in 34%, 41% and 26% respectively of diabetic adolescents with no significant difference between T1DM and T2DM patients. Male gender, T2DM, puberty and the presence of anxiety symptoms were associated with deterioration in glycaemic control post-lockdown. There was a deterioration in HbA1c among male and T2DM patients by 0.76% and 0.94% respectively (p-value 0.013, 0.004 respectively). An increment of HbA1c pre- and post-lockdown was observed in patients with anxiety symptoms (9.39 ± 0.49 vs 10.16 ± 0.54%, p-value 0.028). Patients who displayed stress symptoms showed improvement in their HbA1c (10.00 ± 0.57% vs 9.50 ± 0.063%, p-value 0.036). Conclusion: There is a high prevalence of psychological disturbance among diabetic adolescents during the COVID-19 pandemic, particularly the anxiety component related to poor glycaemic control. Follow-up psychological assessment and support must be given to these young diabetic patients.