As no treatment method has a sufficient level of evidence of success in the treatment of gastric cancer with peritoneal metastasis, there is an urgent need to develop a new treatment that takes into account the pathophysiology of the disease. A novel multidisciplinary treatment approach combining intraperitoneal (IP) paclitaxel (PTX), systemic chemotherapy, and gastrectomy is one of the promising treatment options. We designed a combination chemotherapy regimen of S-1, weekly intravenous and IP PTX and determined the recommended dose in a phase I study. In the phase II study, the median survival time (MST) was 23.6 months. Out of 100 patients treated with this regimen, we performed gastrectomy on 62 patients after disappearance or obvious shrinkage of the peritoneal metastasis, and the MST was 34.5 months. Currently, we are carrying out a phase III trial (PHOENIX-GC trial) comparing our IP regimen to S-1 plus CDDP.