Background: Intraperitoneal paclitaxel is proved to be efficient for peritoneal metastasis of gastric cancer. It remains uncertain the efficacy and safety of the triplets regimen which combined intraperitoneal high-dose paclitaxel with systemic SOX in gastric cancer patients with peritoneal metastasis. This study aimed to evaluate the efficacy and safety of intraperitoneal administration of high-dose paclitaxel, intravenous oxaliplatin and S-1 in patients with peritoneal metastatic gastric cancer.Methods: This single-center, prospective, single-arm phase II study was conducted between January 2017 and May 2019 in West China Hospital, Sichuan University. Patients diagnosed with primary gastric cancer by histopathology and confirmed synchronous peritoneal metastasis were enrolled. This study aimed to evaluate efficacy and safety of intraperitoneal administration of high-dose paclitaxel (80 mg/m 2 , d1), intravenous oxaliplatin (100 mg/m 2 , d1), and S-1 (80 mg/ m 2 , d1-14) of patients. The primary endpoint was 1-year overall survival rate, and the second endpoints were progression-free survival (PFS), overall survival (OS), overall response rate (ORR), disease control rate (DCR) and adverse events. Results:In this single-arm phase II clinical trial, 49 patients received SOX combined intraperitoneal high-dose paclitaxel treatment. One-year survival rate was 81.6% (95% CI, 68.6-90.0%). Median PFS and OS were 6.50 months (95% CI, 2.89-10.11) and 16.9 months (95% CI, 13.58 to 20.22), respectively; ORR was 55.3% (95% CI, 41.3-68.6) and DCR was 76.6% (95% CI, 62.8-86.4). Thirteen patients underwent second laparoscopic detection, but only nine ultimately underwent radical gastrectomy. Subgroup analysis showed that sPCI ≤12 was a good index for a favorable prognosis. The most frequent grade 3/4 toxicities were neutropenia (40.8%), anemia (22.4%), leukopenia (18.4%), nausea (14.3%), and How to cite this article: Tu L, Zhang W, Ni L, et al. Study of SOX combined with intraperitoneal high-dose paclitaxel in gastric cancer with synchronous peritoneal metastasis: A phase II single-arm clinical trial.