Gastric cancer is a common cancer of the gastrointestinal tract and the second most prevalent cause of cancer-associated mortality globally. Gastric cancer-associated mortality is increased in China compared with that in other countries. Key contributors to the poor prognosis of gastric cancer include late clinical presentation and genetic heterogeneity. Treatment based on the subtype of gastric cancer is important for effective therapy. The overexpression of the erb-b2 receptor tyrosine kinase 2 (ERBB2) gene and protein is associated with gastric cancer in humans. Chemotherapy and targeted therapy may control tumor growth and recurrence, which is an important function of conversion surgery. The present study reported a patient diagnosed with gastric cancer with multiple abdominal cavity and retroperitoneal lymph node metastases. ERBB2 amplification and overexpression were identified in both case reports presented. The patients were treated with four cycles of oxaliplatin, capecitabine and trastuzumab. Computed tomography revealed the lymph node metastases decreased in size following treatment, and surgical resection was performed. The four cycles of oxaliplatin, capecitabine and trastuzumab were continued subsequent to surgical resection at the administered dose. No recurrence was observed for >1 year after surgery. Trastuzumab combined with oxaliplatin and capecitabine as a conversion therapy regime for ERBB2-overexpressing advanced gastric adenocarcinoma increased the likelihood of successful surgical resection, and prolonged progression-free survival.