We report herein a 63-year-old female with gastric duplication cyst (GDC), of which resected specimen was histologically shown to be composed of gastric foveolar epithelium and thin bundles of smooth muscle. Computed tomography revealed a thin-walled cystic lesion surrounded by the pancreatic tail, spleen, left kidney, and the stomach. Magnetic resonance imaging demonstrated a thin layer between the cyst and either the spleen or kidney, successfully excluding the possibility that the cyst originated from these organs. Endoscopic ultrasonography failed to show a smooth muscle bundle in the cyst wall, which is a diagnostic finding for GDC. Even retrospectively, these preoperative findings could not distinguish GDC from pancreas-originating cystic lesions. Despite the recent advances in diagnostic imaging modalities, preoperative diagnosis of GDC in adults remains difficult due in part to its rarity and the absence of characteristic findings.