A 64-year-old man underwent abdominal computed tomography (CT) as periodic follow-up following a distal gastrectomy with lymphadenectomy for gastric cancer and mucosal-associated lymphoid tissue (MALT) lymphoma conducted 31 months earlier. Contrast-enhanced CT demonstrated a well-circumscribed mass lesion with heterogeneous density measuring 2.2 cm in diameter located between the second segment of the duodenum and uncinate process of the pancreas. Esophagogastroduodenoscopy revealed no remarkable findings in the remnant stomach; however, the scope could not reach the duodenum due to altered anatomy by Roux-en-Y reconstruction after the distal gastrectomy. The patient underwent surgical resection of the mass lesion under the clinical diagnosis of MALT lymphoma relapse. An orange calculus was apparent in the thinly extended duodenal wall on stretching, and the hall was closed by meticulous primary suture after the duodenal resection. Macroscopically, the extracted calculus was solid and quite hard, measured 2.2 x 2.1 x 2.1 cm, and the cut surface revealed a layered structure in the outer areas with granulated contents in the center. Although duodenal diverticula are relatively common, an enterolith developing within a juxtapapillary duodenal diverticulum is rare, and to the best of our knowledge, this is the first such case due to altered anatomy after gastrectomy reported in the English literature.