Hyponatremia is a dangerous electrolyte disturbance in patients on chemotherapy and may cause sudden death if not detected early. SIADH (syndrome of inappropriate antidiuretic hormone) is one of the known causes of hyponatremia in patients undergoing chemotherapy. Few chemotherapeutic agents, however, are reported to cause SIADH. The current study reports that SIADH developed in a 55 year old woman on S-1 (80 mg/m2) and cisplatin (60 mg/m2) chemotherapy for the peritoneal metastasis of gastric cancer. The patient underwent a total gastrectomy, a splenectomy, and a segmental resection of the transverse colon for gastric cancer. She had used thiazide and betablocker to treat hypertension for 12 years. She admitted to our hospital with complaining of general weakness, dysarthria, loss of appetite, and urinary discomfort. The serum level of sodium and potassium were 94 mEq/L and 2.2 mEq/L respectively. The hyponatremia completely resolved uneventfully after 3% saline infusion, which led to normalized electrolyte balance. The patient was discharged on the 13th hospital day.