Gastric cancer is the fourth most common cancer worldwide and the leading cause of tumor-related death in China. Gastric cancer is a heterogeneous disease and therefore requires different treatments based on the subtype. We describe a patient who had gastric cancer with liver metastases. Biopsy and tumor analysis using the HercepTest revealed a human epidermal growth factor receptor 2 (HER2)-positive adenocarcinoma as confirmed by fluorescence in situ hybridization. The patient was treated with a regimen of trastuzumab, oxaliplatin, and S-1 (six cycles). When positron emission tomography findings suggested that the metastases had resolved, the patient underwent surgery. Histopathologically, no cancer cells were observed in the resected hepatic tissue. The patient underwent tumor resection surgery, during which the tumor and gastric lymph nodes with lesions were removed. The patient has remained disease-free for 3 months. Therefore, trastuzumab may be an effective agent in the chemotherapeutic treatment of liver metastases in patients with HER2-positive gastric adenocarcinoma.