Trastuzumab has recently been recommended for the treatment of epidermal growth factor receptor-2 (HER2)-positive advanced gastric cancer in combination with the capecitabine/cisplatin (XP) versus continuous infusion of 5-fluorouracil/cisplatin (FP) regimen. However, it is unclear whether it is rational to combine trastuzumab with other chemotherapy regimens in clinical practice. Our study demonstrates that adding trastuzumab to oxaliplatin, a commonly used third-generation platinum derivative, increases the antitumor effect in vitro. In MTT assays, combination treatment with oxaliplatin and trastuzumab significantly decreased the concentration of oxaliplatin required to induce 50% growth inhibition in HER2-positive gastric cancer cells. Further investigation revealed that the trastuzumab-oxaliplatin combination induced cell cycle arrest and decreased expression of both p-AKT and p-ERK. Notably, this treatment combination induced downregulation of the excision repair cross-complementation group 1 (ERCC1) protein, which is involved in the key repair process of the oxaliplatin-DNA platinum adduct at the protein level. Similar changes were also observed in HER2-positive breast cancer cells. These findings suggest that trastuzumab synergizes the cytotoxic effect of oxaliplatin on HER2-positive gastric and breast cancer cells. Our study provides preclinical evidence for the optimization of this combination regimen in the treatment of HER2-positive gastric cancer patients.