Here, we report a patient with gastric adenosquamous carcinoma (ASC) with human epidermal growth factor receptor-2 (HER2) overexpression who was successfully treated with trastuzumab-based chemotherapy. The patient was a 66-year-old man preoperatively diagnosed with gastric adenocarcinoma with no evidence of distant metastases. On histopathological examination, the curatively resected tumor was identified as ASC with mixed adenocarcinoma and squamous cell carcinoma components. Multiple liver metastases developed 2.5 months after surgery. Because immunohistochemical staining for HER2 was strong in both components, combination chemotherapy with capecitabine, cisplatin, and trastuzumab was initiated. A partial response was confirmed after 6 treatment cycles and PET and CT scans performed after 13 cycles revealed disease resolution with no uptake in the metastatic lesions. No evidence of disease progression has been observed 16 months after initial chemotherapy. This report suggests the potential utility of trastuzumab-based chemotherapy for HER2-positive gastric ASC.