Pituitary stalk interruption syndrome (PSIS) involves the occurrence of a thin or absent pituitary stalk, hypoplasia of the adenohypophysis, and ectopic neurohypophysis. Diagnosis is confirmed using magnetic resonance imaging. Patients with PSIS have a variable degree of pituitary hormone deficiency and a wide spectrum of clinical manifestations. The clinical course of the disease in our patient is similar to that of a syndrome of inappropriate antidiuretic hormone secretion. This is thought to be caused by failure in the suppression of vasopressin secretion due to hypocortisolism. To the best of our knowledge, there is no case report of a patient with PSIS presenting with hyponatremia as the first symptom in Korean children. Herein, we report a patient with PSIS presenting severe recurrent hyponatremia as the first symptom, during adolescence and explain the pathophysiology of hyponatremia with secondary adrenal insufficiency.