Metastatic brain tumors from gastric cancer are extremely rare. A 61-year-old Korean woman, initially presenting with polydipsia and polyuria, was found to have metastatic lesions in the brain by MRI. We performed several diagnostic procedures to determine the origin of the brain metastases. She was revealed to have a soft tissue mass of the right adrenal gland and fungating ulcers in the stomach. Histologic studies of both the adrenal gland mass and gastric tissues revealed malignant tumors composed of anaplastic cells. Based on the electron microscopy study, the malignant tumor of the right adrenal gland was a metastatic lesion from the anaplastic carcinoma of stomach. Therefore, the malignant tumors of the brain were assumed to have originated from the gastric cancer. This case report is presented to make clinicians aware of the possibility that diabetes insipidus (polydipsia) may present as an initial manifestation of brain metastases.