SummaryFulminant type 1 diabetes (FT1D) is a subtype of diabetes characterized by rapid progression of β-cell destruction, hyperglycemia, and diabetic ketoacidosis (DKA). The pathogenesis of this disease remains unclear. However, viral infections, HLA genes, and immune checkpoint inhibitor use were reportedly involved in this disease. A 51-year-old Japanese man with no chronic medical condition was admitted to our hospital with complaints of nausea and vomiting. Cough, sore throat, nasal discharge, and diarrhea were not noted. He had a medical history of at least two influenza infections. His vaccination history was notable for receiving an inactive split influenza vaccine 12 days prior to developing these symptoms. He was diagnosed with DKA associated with FT1D. His HLA class II genotypes were nonsusceptible to FT1D, and he had a negative history of immune checkpoint inhibitor use. The destruction of the pancreas by cytotoxic T cells is reported to be involved in FT1D. Inactive split influenza vaccines do not directly activate cytotoxic T cells. However, these could activate the redifferentiation of memory CD8-positive T cells into cytotoxic T cells and induce FT1D, as this patient had a history of influenza infections.Learning pointsInfluenza split vaccination could cause fulminant type 1 diabetes (FT1D).The mechanism of influenza split vaccine-induced FT1D might be through the redifferentiation of CD8-positive memory T cells into cytotoxic T cells.