A 58-year-old woman with a nine-year history of type 2 diabetes mellitus began vomiting after lunch one day in October 2011. She had no other symptoms and subsequently visited our hospital. The vomiting persisted even after the administration of antiemetic drugs, and she was therefore admitted under a diagnosis of acute gastroenteritis. Neither computed tomography of the abdomen or head nor upper endoscopy revealed any abnormalities, and there was no acidosis. However, brain magnetic resonance imaging (MRI) performed on Day 4 revealed vasogenic edema in the bilateral occipital lobes. We thus diagnosed the patient with posterior reversible encephalopathy syndrome (PRES). After admission, her blood pressure remained between 150/70 mmHg and 180/90 mmHg. In order to treat the PRES, we kept her blood pressure under 140/90 mmHg with intravenous nicardipine. Thereafter, her vomiting gradually diminished, ultimately disappearing by Day 7. Brain MRI performed on Day 10 showed no abnormalities, and she was discharged on Day 11. Although the number of reports of PRES associated with diabetes mellitus is increasing, no previous studies have described a case of PRES in a patient with diabetes mellitus presenting with only repetitive vomiting, no headaches, triggered by mild hypertension. This case suggests the importance of considering PRES in the differential diagnosis of repetitive vomiting in diabetic patients with mild hypertension.