Gastric foregut duplication cyst is a rare congenital anomaly. There have been very few reports of bronchogenic cysts in the abdominal cavity attached to the stomach. We report a case of a 57-year-old Korean man with an adenocarcinoma arising within a bronchogenic cyst of the stomach. Pathologic findings revealed ciliated columnar epithelium lining the cyst and a solid papillary adenocarcinoma involving the subserosa of the stomach and the skeletal part of the diaphragm. Surgical resection is the treatment of choice for foregut cysts of the stomach, and the possibility of malignancy should be considered in patients with gastric duplication cysts.