Metastatic tumors of the pituitary are uncommon and usually asymptomatic. They are often incidental findings from imaging workups for other medical issues or from the assessment of primary tumors in other locations. Diabetes insipidus is the most common symptom resulting from pituitary tumors, including pituitary metastases. A 56-year-old woman with primary lung adenocarcinoma underwent video-assisted thoracic bilobectomy. Regular follow-up was unremarkable until 15 months after surgery, when she presented with polyuria and polydipsia suggestive of diabetes insipidus. A pituitary mass was found on brain magnetic resonance imaging; the diagnosis of lung adenocarcinoma metastasized to the pituitary was confirmed by trans-sphenoidal surgery and biopsy of the pituitary mass. Diabetes insipidus and hormonal profiles are the key to recognize the existence of pituitary metastases, and patients with primary lung cancers presenting with diabetes insipidus should be evaluated for pituitary metastases.