This is an open-label, international, two-arm, parallel, randomized, Phase 3 study evaluating the efficacy and safety of S-1/cisplatin versus 5-FU/cisplatin in patients with advanced gastric cancer previously untreated with chemotherapy for advanced disease. Patients will be randomly assigned (1:1) to S-1/cisplatin (experimental arm) or 5-FU/cisplatin (control arm). Patients will be stratified by number of metastatic sites (one vs. more than one), locally advanced or metastatic disease, prior adjuvant therapy (yes or no), measurable or non-measurable disease, and center.