BACKGROUND It is well known that diabetes mellitus contributes to COVID-19 severity. Recently, there have been reports of an increase in the number of children with type 1 diabetes after the COVID-19 pandemic. CASE REPORT A 52-year-old woman presented to the Emergency Department with disturbance of consciousness, accompanied by a 1-day history of thirst, a fever of 38°C, and breathlessness. She had a positive coronavirus antigen test. Her initial vital signs assessment showed a heart rate of 120 beats per minute, blood pressure 90/50 mmHg, temperature 37.3°C, and respiratory rate 30 breaths/minute with an oxygen saturation of 100% with 10 L oxygen inhalation. Her initial laboratory test results showed a blood glucose level of 1507 mg/dl, HbA1c of 10.1%, ketone 2+, and blood gas pH 7.113. The patient was diagnosed with diabetic ketoacidosis (DKA). There were mild inflammatory findings with blood CRP 0.14 mg/dl and a white cell count of 12 400/μL, but no pneumonia on a chest CT scan. Therefore, the patient was diagnosed with COVID-19 and DKA. The patient was positive for anti-glutamic acid decarboxylase (anti-GAD antibody) and had markedly low levels 24-h urine C-peptide (CPR). She was diagnosed with acute-onset type 1 diabetes mellitus, as her blood examination showed a postprandial blood glucose level of 100 mg/dl and HbA1c of 5.7% 2 months before admission. After admission, fluid replacement and continuous intravenous insulin infusion therapy were started, and blood glucose and blood gas pH improved over 10 h. CONCLUSIONS There have been reports of cases of type 1 diabetes consequences of COVID-19, but the mechanism has not been elucidated.