Gastric duplication cysts are rare congenital anomalies, and malignant transformation has only been reported in 11 cases. A healthy 57-year-old woman presented with abdominal discomfort, and computed tomography revealed a 5.8 × 6.6 × 8.2 cm mass at the gastric fundus. On endoscopic ultrasound, the mass was mostly hypoechoic with anechoic cystic cavities arising from the submucosal layer. Fine-needle aspiration was suspicious for adenocarcinoma. Surgical pathology confirmed high-grade adenocarcinoma, clear cell cytology arising from a foregut duplication cyst. Endoscopic ultrasound is underutilized in the evaluation duplication cysts and should be considered in routine workup.