The pituitary gland and infundibulum can be involved in a variety of medical conditions, including infiltrative diseases, fungal infections, tuberculosis, and primary and metastatic tumors. Metastases to the pituitary gland are absolutely rare, and they are generally secondary to pulmonary carcinoma in men and breast carcinoma in women. Pituitary metastases more commonly affect the posterior lobe and the infundibulum than the anterior lobe. The posterior lobe involvement may explain why patients with pituitary metastases frequently present with diabetes insipidus. We are presenting a case report of a 78-year-old male patient who had metastatic prostate with sudden onset of polyuria and persistent thirst. He had no electrolyte imbalance except mild hypernatremia. The MRI scan of the brain yielded a suspicious area in pituitary gland. A pituitary stalk metastasis was found on magnetic resonance imaging (MRI) of pituitary. Water deprivation test was compatible with DI. A clinical response to nasal vasopressin was achieved and laboratory results revealed central diabetes insipidus. As a result, the intrasellar and suprasellar masses decreased in size, and urinary output accordingly decreased.