Background: S-1, an oral fluoropyrimidine, plus cisplatin (SP) regimen is one of the standard chemotherapy as first-line for advanced gastric cancer (AGC). Although ToGA study demonstrated that trastuzumab (T-mab) in combination with capecitabine plus cisplatin or fluorouracil plus cisplatin improved the overall survival of patients (pts) with HER2-positive AGC, there was no study evaluating the efficacy and the safety of T-mab in combination with SP regimen. Methods: Eligibility criteria included gastric or esophagogastric junction adenocarcinoma; HER2-positive confirmed by IHC and/or FISH (IHC 3+ or IHC 2+ and FISH positive); unresectable or recurrent; measurable lesion; no history of chemotherapy or radiotherapy; age≤75; ECOG PS of 0-1; and adequate organ function. Pts received S-1 at 40–60 mg depending on body surface area, po bid, day 1-14, and cisplatin 60 mg/m², iv, day 1, plus T-mab 8 mg/ kg, iv, day 1 (6 mg/ kg, iv, d1 from 2nd course), repeated every 3 weeks until disease progression. Primary endpoint was response rate assessed by the RECIST (ver 1.1). The planned sample size was 50 based on the threshold response rate of 35%, the expected rate of 50%, power of 80%, and 1-sided α of 0.1. Results: A total of 56 pts were enrolled from July 2011 to May 2012. Two pts were ineligible with inadequate renal function and no measurable lesion. Characteristics of 54 eligible pts were as follows: median age of 66 (range 34-75), M/F: 42/12, PS0/1: 42/12, unresectable/recuurent: 51/3, and IHC 2+/3+: 9/45. As one patient did not receive the protocol treatment due to the rapid progression of tumor, the efficacy and the safety analyses were conducted in the full analysis set of 53 pts. The confirmed response rate assessed by the independent review committee was 68%, and the disease control rate was 94%. The response rate without interval confirmation was 75%. The grade 3/4 adverse events (>5% of pts) were as follows: neutropenia 30%, leucopenia 8%, anorexia 21%, diarrhea 8%, hypoalbuminemia 8%, vomiting 6%, and increased creatinine 6%. Conclusions: T-mab in combination with triweekly SP regimen showed promising antitumor activity and manageable toxicities in pts with HER2-positiveAGC. Clinical trial information: UMIN000005739.