Background: Intraperitoneal (IP) chemotherapy is a promising treatment option for gastric cancer with peritoneal metastasis. Although a phase III study failed to show a statistically significant superiority of IP paclitaxel (PTX) combined with S-1 and intravenous PTX over S-1/cisplatin (SP), the standard of care as a first-line treatment in Japan, the sensitivity analysis suggested clinical efficacy of the IP PTX. Thus, attempts to combine IP PTX with other systemic therapies with higher efficacy have been warranted. After a dose-finding study, we sought to explore efficacy of a new regimen that combined IP PTX with SP. Methods: Gastric cancer patients with peritoneal metastasis confirmed by diagnostic imaging, laparoscopy or laparotomy were enrolled in the phase II multi-institutional prospective trial. In addition to the established SP regimen (S-1 administered orally at a dose of 80 mg/m² bid for 21 days followed by a 14-day rest and cisplatin administered intravenously at a dose of 60 mg/m² on day 8), IP PTX was administered on days 1, 8 and 22 at a dose of 20 mg/m². The primary endpoint is overall survival (OS) rate at one year after treatment initiation. Secondary endpoints are progression free survival (PFS), response rate and toxicity. Results: Fifty-three patients were enrolled and fully evaluated for OS and toxicity. The median number of courses was 7 (range 1-20). The 1-year OS rate was 74% (95% CI, 60-83%). The median survival time was 19.4 months (95% CI, 16.7 months-). The 1-year PFS rate was 57% (95% CI, 42-69%). The overall response rate was 20% (95% CI, 1-72%) in 5 patients with target lesions. Cancer cells ceased to be detected by peritoneal cytology in 23 (64%) of 36 patients. Fourteen (26%) patients underwent gastrectomy after response to chemotherapy. The incidences of grade 3/4 hematological and non-hematological toxicities were 43% and 47%, respectively. The frequent grade 3/4 toxicities included neutropenia (23%), anemia (29%), diarrhea (13%) and anorexia (17%). Intraperitoneal catheter and implanted port-related complications were observed in 4 patients. There was 1 treatment-related death. Conclusions: IP PTX combined with SP is well tolerated and active in gastric cancer patients with peritoneal metastasis. Clinical trial information: UMIN000023000.