Objective: To describe the occurrence of metastatic malignant insulinoma in a patient with preexisting type 2 diabetes mellitus. Methods: We present a detailed case report, with clinical, biochemical, and imaging findings, and summarize the data from 21 similar cases in the literature. Results: The occurrence of malignant insulinoma in a patient with preexisting diabetes is very rare and thus can be a diagnostic challenge. In our patient with type 2 diabetes, endogenous hyperinsulinism was confirmed by demonstrating elevated insulin and C-peptide levels during hypoglycemic episodes in the absence of sulfonylurea on a blood screen. Abdominal computed tomographic scan and magnetic resonance imaging revealed a pancreatic mass as well as metastatic lesions in the liver. The pancreatic mass was removed and confirmed to be a malignant insulinoma. This procedure was followed by disappearance of the hypoglycemic episodes as well as the diabetes for a few months. On follow-up, however, more metastatic lesions appeared in conjunction with a protracted course of hypoglycemia that necessitated treatment with antihypoglycemic agents and, 3 years after the initial surgical intervention, culminated in the death of the patient. Conclusion: Our patient is one of the few subjects known to have a malignant insulinoma in conjunction with preexisting diabetes. A high degree of suspicion for the presence of an insulinoma should be maintained when unexplained hypoglycemic episodes occur in a patient with previously stable diabetes.