Hypoglycemia is seen with increasing frequency after bariatric surgery. After the diagnosis of hypoglycemia has been clarified, malnutrition, drugs, hormone deficiencies, insulinoma, extra-islet tumors, post-bariatric hypoglycemia (PBH), early or late dumping syndrome, and nesidioblastosis should be considered in the differential diagnosis. A few case reports of insulinomas presenting after bariatric surgery have been reported in the literature. The coexistence of insulinoma and type 2 diabetes mellitus (T2D) is very rare. We herein report a clinical case of insulinoma presenting with severe hypoglycemia in a patient with a history of gastric transit bipartition.A patient with type 2 diabetes mellitus underwent gastric transit bipartition surgery due to the inability of medical therapy to provide adequate hyperglycemia control. After the operation, hypoglycemic symptoms appeared, and a reversal operation was performed, considering the diagnosis as PBH. After the reverse operation, the patient's hypoglycemia symptoms did not regress. The patient was admitted to our endocrinology clinic due to the persistence of hypoglycemia and symptoms such as fatigue, palpitation, and syncope. The patient's detailed anamnesis was examined, additional tests were performed, and the patient was diagnosed with insulinoma. The symptoms of hypoglycemia and the need for treatment for diabetes mellitus disappeared after the Whipple operation.This is the first case of insulinoma after gastric transit bipartition and subsequent reversal operations. In addition, the patient's diagnosis of diabetes mellitus makes this case unique. Although this is a very rare case, clinicians must be aware of it, especially if the patient has hypoglycemic symptoms during the fasting state.