BACKGROUND Duodenal adenocarcinomas are relatively rare. We report here a particularly rare case of duodenal adenocarcinoma that arose within a duodenal bulb diverticulum and was successfully managed surgically. METHODS A 70-year-old woman presented to us with a history of duodenal bulb ulceration and a diverticulum in the same area diagnosed by esophagogastroduodenoscopy. Routine endoscopic examination revealed an elevated lesion growing from within the diverticulum to outside of it. Examination of a biopsy specimen resulted in a diagnosis of well-differentiated adenocarcinoma. Computed tomography failed to detect any lesions in the duodenal bulb. No enlarged lymph nodes or distant metastases were found. Fluoroscopic examination revealed a pool of contrast suggestive of a diverticulum in the duodenal bulb on the anal side of the pyloric ring. Thus, the clinical stage according to the TNM Classification based on the 8th edition of the Union for International Cancer Control/American Joint Committee on Cancer (UICC/AJCC 8th edition) was T1N0M0: Stage I. Endoscopic resection was not feasible because the tumor's origin was within the diverticulum. Furthermore, surgical local resection was considered not to be feasible because of the tumor size and location. We performed laparoscopic distal gastrectomy, D1+ lymph node dissection, and Roux-en-Y reconstruction. The patient was discharged on the 10th postoperative day and is currently on outpatient follow-up with no evidence of recurrence 1 year postoperatively. CONCLUSIONS We report here a rare case of an adenocarcinoma arising within a duodenal bulb diverticulum. We successfully performed a laparoscopic distal gastrectomy, this procedure being considered the optimal surgical approach for this patient.