S-1 has been developed as a new oral anticancer drug, based on the biological modulation of 5-fluorouracil. We report a patient with highly advanced gastric carcinoma who was treated successfully with a new combination chemoradiotherapy using S-1 and cisplatin (CDDP). The patient was a 37-year-old man who was diagnosed with advanced gastric carcinoma (T4N3M0) that had invaded the diaphragm and the paraaortic tissues. Remarkable tumor reduction was observed in the primary tumor and metastatic lymph nodes around the stomach after three cycles of the therapy. Radiological examination before surgery determined that a partial response (PR) had been achieved by the initial therapy. Adverse effects included only a gastrointestinal disorder that was limited to grade 2 when low-dose CDDP was utilized in the regimen, while an initial high dose of CDDP resulted in grade 3 toxicity, due to myelosuppression. The patient underwent curative surgery, including total gastrectomy, D2 lymph node dissection, and splenectomy, after completion of the radiochemotherapy regimen. No surgical complication was observed. No tumor cells were detected by pathological evaluation of the resected stomach and all the regional lymph nodes, confirming a pathological complete response (CR; grade 3). This regimen is a potent treatment for advanced gastric carcinoma, especially when used as preoperative chemotherapy to control cancer cells.