A 59-year-old man with confused mental status was admitted to our hospital. Laboratory reports showed him to have severe hyponatremia, and additional studies revealed panhypopituitarism. Brain magnetic resonance imaging showed a sellar cystic lesion, which consisted of a Rathke cleft cyst. Thus, the mass effect of the Rathke cleft cyst resulted in panhypopituitarism and finally induced euvolemic hyponatremia. On the basis of these results, supplementation with thyroid hormone and glucocorticoid was started, and the patient’s serum sodium level was gradually corrected and maintained within the normal range. Here, we report this case of euvolemic hyponatremia caused by a Rathke cleft cyst.