A 65-year-old Japanese male with type 2 diabetes mellitus was admitted to our hospital with a productive cough and worsening dyspnea. He had started receiving vildagliptin, which is one of the dipeptideylpeptidase-4 (DPP-4) inhibitors, several days before the appearance of his symptoms. Laboratory findings revealed markedly elevated levels of immunoglobulin E and Krebs von den Lungen-6. Chest computed tomography revealed ground-glass opacity with irregular reticulation throughout both lungs. Biopsy specimens by transbronchial lung biopsy showed subacute interstitial pneumonia and an organizing pneumonia pattern with acute alveolar injury. The drug lymphocyte stimulation test showed a positive result for vildagliptin. Withdrawal of vildagliptin and administration of glucocorticoid treatment improved his respiratory condition and radiological findings. Therefore, we diagnosed the patient with vildagliptin-induced interstitial pneumonia based on both his clinical course and pathological findings. Interstitial pneumonia as a side effect of vildagliptin is rare. It may be necessary to monitor the respiratory condition of patients upon administration of DPP-4 inhibitors until further evidence is obtained.