Fulminant type 1 diabetes mellitus (FT1DM) is a subtype of type 1 diabetes (T1DM) with an acute onset. There are limited reports on FT1DM in pediatric patients. Here, we report the case of a Japanese female infant with FT1DM, representing the youngest female with FT1DM documented to date. The patient was referred to our hospital at 10 mo of age. Although her laboratory findings met the diagnostic criteria for severe diabetic ketoacidosis, her HbA1c level was not excessively high. Anti-glutamic acid decarboxylase and anti-insulinoma-associated protein-2 antibodies were not detected. Test results for insulin autoantibodies were positive. The glucagon stimulation-loading test revealed a C-peptide level of < 0.6 ng/mL. At 8 yr of age, the patient was diagnosed with Graves' disease. Human leukocyte antigen typing and analysis of a single-nucleotide polymorphism (rs3782151) in CSAD/lnc-ITGB7-1 revealed that the patient was predisposed to FT1DM owing to these two factors. Her findings at the disease onset fulfilled the diagnostic criteria for FT1DM. Although rare in FT1DM, the patient developed Graves' disease, a complication commonly associated with autoimmune T1DM. Moreover, although her condition at onset and genetic predisposition were consistent with those of FT1DM, her clinical course resembled that of autoimmune T1DM.