The breast and ovarian cancer susceptibility gene 1 (BRCA1) is a well-known tumor suppressor gene implicated in the predisposition of early-onset breast and ovarian cancer. The aim of this study is to identify the prognostic significance of BRCA1 in patients with gastric cancer undergoing surgery and platinum-based adjuvant chemotherapy. BRCA1 protein expression profiles were evaluated by immunohistochemistry (IHC) on surgical specimens of 637 gastric cancer patients. The relationships between BRCA1 expression and existing prognostic factors, platinum-based adjuvant chemotherapy, disease-free survival, and overall survival were analyzed. There were 637 stage II/III patients, of which 219 samples (34 %) were evaluated as BRCA1 IHC positive. BRCA1 expression had statistically significant association only with tumor differentiation (p = 0.004). The patients with BRCA1-positive expression had significantly prolonged overall survival (p = 0.049). The patients received platinum-based adjuvant chemotherapy showed a better prognosis (p = 0.017). The patients with BRCA1-negative expression benefited more from platinum-based adjuvant chemotherapy (p = 0.024). Multivariate analysis demonstrated that tumor size, T category, N category, vascular or nerves invasion, and platinum-based adjuvant chemotherapy were good prognostic factors for overall survival (p < 0.05). BRCA1 expression was not significantly related to overall survival (p = 0.127). The positive correlation between BRCA1 expression and overall survival suggests that patients with BRCA1 expression have a better prognosis in gastric cancer. Patients without BRCA1 expression can benefit from platinum-based adjuvant chemotherapy. However, BRCA1 expression might not be a good prognostic factor.