Type 1 diabetes mellitus is classified as either autoimmune or idiopathic. Fulminant type 1 diabetes was originally reported as a subtype of idiopathic type 1 diabetes. Though involvement of viral infections has been suggested as a triggering mechanism, its pathogenesis remains unknown. Here, we present a case of fulminant type 1 diabetes associated with significant elevation of mumps titers. A 56-year-old Japanese man had suffered from nausea and generalized fatigue for two days before being transferred to our hospital in a confused state. Findings on admission revealed a high blood glucose level, near-normal HbA1c level, metabolic acidosis, and increased urinary ketone levels. Serum tests for islet-associated autoantibodies were negative. The serum, urinary C-peptide levels and the result of glucagon test indicated severe impairment of insulin secretion. These results were compatible with the diagnosis of fulminant type 1 diabetes. Also, he was suspected as having mumps infection on the basis of serological testing. These findings suggest that fulminant type 1 diabetes developed after mumps virus infection in our case. To the best of our knowledge, no other report has indicated an association between a recent mumps infection and the onset of fulminant type 1 diabetes. This case suggests an association between fulminant type 1 diabetes and mumps virus infection.