Background: A combination of S-1 and cisplatin (SP) has been the standard regimen for advanced gastric cancer (AGC) in East Asia. The combination of S-1 and oxaliplatin (SOX100) was demonstrated to be non-inferior to SP in the randomized phase III study. The ToGA study demonstrated that trastuzumab (T-mab) combination therapies with cisplatin and fluoropyrimidines improved the overall survival of patients with HER2-positive AGC. This multicenter study is the first phase II trial to assess the efficacy and safety of T-mab in combination with S-1 and oxaliplatin (HER-SOX130) in HER2-positive AGC. Methods: Patients with HER2-positive AGC or recurrent gastric cancer defined to be IHC 3+ or IHC 2+/FISH positive received 80 mg/m² S-1 per day orally on days 1–14, 130 mg/m² oxaliplatin intravenously on day 1, and T-mab (8-mg/kg loading dose and 6 mg/kg thereafter) intravenously on day 1 of a 21-day cycle until one of the criteria for withdrawal of the study treatment occurred. The primary end-point was the response rate (RR). Adverse events were recorded based on the NCI-CTCAE Vers.4.0. The threshold response rate was defined as 50%, and the expected rate was set at 70%, with an 80% power and a 1-sided alpha value of 0.05. The calculated sample size was 37 patients. Results: For this study, 42 patients (median age, 66 years) were enrolled from June 2015 to May 2016. Three patients were excluded owing to ineligibility. Efficacy and safety analyses were conducted in the full analysis set of 39 patients. The proportion of patients with IHC 3+ was 87%. The confirmed RR assessed by the independent review committee was 82.1(32/39) % (95% confidence interval [CI]: 67.3–91.0), and the disease control rate was 87.2(34/39) % (95% CI: 73.3–94.4). The incidence rates of grade 3 or 4 adverse events were as follows: neutropenia, 12.8%; thrombocytopenia, 17.9%; anemia, 10.3%; sensory neuropathy, 5.1%; anorexia, 17.9%; diarrhea, 7.7%; and teary eyes, 2.6%. Conclusions: HER-SOX130 demonstrated encouraging efficacy with a favorable safety profile. The survival benefit of this regimen needs to be validated by conducting further follow-up of patients. Clinical trial information: 000017552.