Background: In Japan, S-1 plus cisplatin is regarded as one of the standard first line treatment of advanced gastric cancer (AGC). However, the prognosis of AGC remains dismal. The development of more effective chemotherapeutic regimen is thus warranted. A combination of irinotecan, cisplatin, and S-1 (IPS) can be a promising triplet therapy for advanced gastric cancer. We conducted a phase I study of IPS to determine the dose-limiting toxicity (DLT), maximum-tolerated dose (MTD) and recommended dose (RD), and to assess its safety and antitumor activity in patients with AGC. Methods: This phase I study was designed and conducted in a 3 + 3 manner to determine the recommended dose (RD) of IPS for the subsequent phase II study. Patients received an escalating dose of intravenous irinotecan (level 1: 100/level 2: 125/level 3: 150 mg/m²) on day 1, a fixed dose of intravenous cisplatin (60 mg/m²) on day 1, a fixed dose of S-1 (80 mg/m² b.i.d.) orally on days 1-14, every 4 weeks. Results: Twelve patients were enrolled between June 2013 and February 2017. During the first cycle, one of the six patients in level 1 and two of six patients in level 2 developed the DLT (grade 4 leucocytopenia and grade 3 febrile neutropenia). The MTD of irinotecan was 125 mg/m 2 (level 2) and the RD of irinotecan was considered to be 100 mg/m² (level 1). The most common grade 3 or 4 adverse events included neutropenia 75 % (9/12), anemia 25% (3/12), anorexia 8% (1/12), and febrile neutropenia 17% (2/12). Among six patients with measurable lesions, the response rate was 66.7% (4/6) [95% CI, 33.3-90.7%]. Two patients were performed R0 resection after IPS, with one patient achieved pathological complete response. The median survival time is under analysis. Conclusions: RD of IPS was determined to be 100 mg/m² of irinotecan, 60 mg/m² of cisplatin, and 80 mg/m² of S-1. Our data showed that this regimen provided acceptable antitumor activity and a favorable toxicity profile. Further evaluation of this regimen is warranted. Clinical trial information: UMIN000006864.