Background Global COVID-19 outbreak has been such a stressful experience for most of the people. Using a web-based cross-sectional study, we aimed to evaluate the acute stress response, depression, and anxiety in patients with diabetes mellitus (DM) during the COVID-19 pandemic, and to examine the effect of these psychiatric problems on diet habits and glycemic controls of patients. Methods This web-based survey of COVID-19 was sent to the patients through the Whatsapp platform. All participants reported their demographic data, diabetes-related information, changes in self-monitoring blood glucose measurements, physical parameters, and eating habits after COVID-19, then completed Hospital Anxiety and Depression Scale (HADS) and the Impact of Event Scale, Revised (IES-R) questionnaires which assessed acute stress sypmtoms, anxiety, and depression. Results Three hundred and four patients with DM [(141 type 1 DM (T1D) and 163 type 2 (T2D)] were included in the study. In our study, female gender, higher BMI and weight, decreased in financial income after outbreak, presence of diabetic complications and comorbid diseases (i.e., retinopathy, neuropathy, diabetic foot, hypertension, dyslipidemia), worsened glycemic levels, increased carbohydrate consumption, and snacking were associated with higher anxiety and depression scores. Depression was higher in patients with T2D and duration of illness was correlated with acute stress level. Conclusions It is important to be aware of the possibility of acute stress, depression, and anxiety after pandemic in patients with DM whose glycemic control is impaired. Psychological problems should not be ignored beyond physical inactivity and worsening eating habits.