Background: Systemic chemotherapy with oral fluoropyrimidine and platinum is recommended for the first-line treatment for unresectable or recurrent gastric cancer in Japan. However, some patients with severe peritoneal metastasis cannot take oral medications because of obstruction or dysfunction of the gastrointestinal tract. FOLFOX is regarded as one of the candidates for the standard of care for gastric cancer, and is now being evaluated in clinical trials. Intraperitoneal (IP) paclitaxel (PTX) provides sustained high local concentrations, and its efficacy has been shown in ovarian and gastric cancer. We developed a regimen combining IP PTX with S-1/PTX, and the phase III trial comparing with S-1/cisplatin suggested efficacy of this regimen. Therefore, we designed a regimen combining IP PTX with mFOLFOX6, and started an exploratory study in gastric cancer patients with peritoneal metastasis and inadequate oral intake. Methods: This is a prospective, multicenter, single-arm phase I/II study. Eligibility criteria include: pathologically proven unresectable or recurrent gastric adenocarcinoma; peritoneal metastasis; inadequate oral intake; adequate bone marrow function; acceptable liver and renal function; ECOG performance status of 0-2 and age between 20-80 years. Patients undergo diagnostic laparoscopy and are implanted with an IP port in the subcutaneous space of the lower abdomen, with a catheter placed in the pelvic cavity. mFOLFOX6 is administered bi-weekly, and IP PTX 20 mg/m² is administered through an IP port on days 1, 8 and 15. The treatment course is repeated every 4 weeks until disease progression or unacceptable toxicity. The primary endpoint is the 1-year overall survival rate. Secondary endpoints are progression free survival, time to treatment failure, response rate, negative conversion rate on peritoneal cytology and safety. Twenty medical institutions from all over Japan participate in this study, and 34 patients are to be enrolled in two years. Toxicity will be evaluated in the early stage, and the protocol will be reconsidered and revised if all of the first 3 or more than 2 of the first 6 patients develop dose-limiting toxicities. Clinical trial information: UMIN000019206.