Background: Intraperitoneal (IP) chemotherapy is a promising treatment option for gastric cancer with peritoneal metastasis. We previously verified the safety and efficacy of IP paclitaxel (PTX) combined with S-1 and intravenous PTX in phase I and phase II studies. S-1 plus oxaliplatin (SOX) demonstrated efficacy in a phase II study, and is regarded as a candidate for the next-generation standard regimen for gastric cancer. We designed a new regimen combining weekly IP PTX with SOX in order to maximize systemic effects as well as local effects in the peritoneal cavity. A dose-escalation study of IP PTX in combination with fixed doses of SOX was carried out to determine the maximum-tolerated dose (MTD) and recommended dose (RD). Methods: PTX was administered intraperitoneally on days 1 and 8 with an initial dose of 20 mg/m² (level 1), stepped up to 30 mg/m² (level 2) or 40 mg/m² (level 3) depending on observed toxicity. S-1 was administered orally at a dose of 80 mg/m²/day (b.i.d.) for 14 days followed by a 7-day rest. Oxaliplatin was administered intravenously at a dose of 100 mg/m² on day 1. This treatment was repeated every 3 weeks. Toxicity was graded according to CTCAE v4.0. Dose-limiting toxicities (DLTs) were defined as grade 4 leukopenia, grade 3 febrile neutropenia, grade 3 thrombocytopenia, and grade 3 non-hematological toxicity. The MTD was defined as the dose level at which 2 or more of 3 or 6 patients developed DLTs during two courses of treatment. The RD was defined as one dose level under the MTD. Results: A total of 12 gastric cancer patients with peritoneal metastasis were enrolled. No DLTs were observed at all dose levels. Neutropenia in one patient at dose level 3 was the only grade 3 toxicity observed. Grade 2/3 leukopenia, neutropenia and thrombocytopenia were observed only in 2 patients at dose level 3. Regarding grade 2 non-hematological toxicities, anorexia, fatigue and nausea were observed in 6, 4 and 2 patients, respectively, independent of dose levels. Consequently, the MTD was not reached, and the RD of IP PTX was determined to be 40 mg/m² (level 3). Conclusions: Combination chemotherapy of IP PTX with SOX was shown to be a safe regimen that should be further explored in clinical trials.