A 35-year-old Japanese woman was admitted with coma following flu-like symptoms. She was diagnosed with diabetic ketoacidosis and fulminant type 1 diabetes (FT1D) and received intravenous infusion of insulin and saline. The next day, the ketoacidosis disappeared, and she recovered consciousness. However, extensive ST-segment elevations in the electrocardiogram appeared with a positive troponin test, and the patient developed pulmonary edema on day 3. An echocardiogram showed globally reduced wall motion of the left ventricle and mild pericardial effusion. Despite medical therapy with intravenous furosemide, carperitide, and catecholamines, her cardiac function deteriorated rapidly, with the left ventricular ejection fraction decreasing to 26% within 7 hours, and progressed to cardiogenic shock that afternoon. The patient received mechanical circulatory support for 4 days with intra-aortic balloon pumping and percutaneous cardiopulmonary support, and recovered fully from circulatory failure. A paired serum antibody test showed a significantly elevated titer against parainfluenza-3 virus, indicating a diagnosis of fulminant viral myocarditis. She was discharged on multiple daily insulin injection therapy, and her subsequent clinical course has been uneventful. In summary, we present a case of concurrent FT1D and fulminant viral myocarditis. Parainfluenza-3 viral infection was confirmed serologically and was considered to be a cause of both the FT1D and fulminant myocarditis.