Some patients with insulinoma present with neuropsychiatric symptoms and are often misdiagnosed with psychiatric disease. We present the case of a 72-year-old Japanese female who exhibited violent behavior while asleep and received a diagnosis of suspected rapid eye movement sleep behavior disorder (RBD). She was admitted to the psychiatry ward after receiving levomepromazine 25 mg intramuscularly. The patient's blood glucose level was 27 mg/dL at the time of hospitalization, and a biochemical examination revealed that her insulin level was 9.1 µU/mL and C-peptide level was 2.16 ng/mL. A contrast-enhanced computed tomography revealed a mass 8 mm in diameter in the pancreatic head. The diagnosis was changed from RBD to insulinoma. The sleep behavior disorder disappeared after continuous glucose administration. After enucleation of the insulinoma, the administration of glucose was discontinued, and her blood glucose levels recovered. This case suggests that insulinoma should be considered by physicians and psychiatrists in the differential diagnosis of patients with symptoms presenting as RBD.