A 59-year-old man complaining of dyspnea, anterior chest oppression, and hypotension was diagnosed to have cardiac tamponade due to massive pericardial effusion. A cytological analysis of the pericardial effusion disclosed adenocarcinoma. An endoscopic study revealed gastric cancer in the lesser curvature wall of the middle body of the stomach, and signet-ring cell carcinoma was confirmed histologically. The gastric cancer was complicated by malignant pericardial effusion, and metastasis to the mediastinal lymph nodes. The patient was treated with pericardiocentesis followed by systemic chemotherapy consisting of TS-1 and cisplatin (CDDP). After 5 months, pericardial effusion disappeared and the primary gastric tumor decreased in size. Our experience suggests that the systemic chemotherapy of TS-1 and CDDP may be effective for controlling advanced gastric signet-cell carcinoma accompanied by malignant pericardial effusion.