Malignant transformation of duplication cyst is a rare condition. The authors report the original case of a degenerated gastric duplication cyst in a 67-year-old patient. The histologic examination revealed a gastric duplication cyst infiltrated with both adenocarcinoma and squamous cell carcinoma. Local carcinomatosis was found at laparotomy. The patient died six months after complete macroscopic resection of the lesion, with metastatic disease.