RATIONALE: Adenomyomatosis is a rare benign condition characterized by epithelial proliferation and diverticular formation in the gastrointestinal tract, most commonly affecting the gallbladder. Its occurrence in the bile duct is uncommon but significant, as it can cause biliary obstruction, cholestasis, and abdominal pain-symptoms that closely mimic early-stage cholangiocarcinoma. This overlap creates diagnostic challenges and increases the risk of misdiagnosis and overtreatment, making accurate identification essential. PATIENT CONCERNS: A 54-year-old woman presented with persistent epigastric dull pain. Imaging studies revealed a dilated common bile duct (CBD), raising concerns about biliary pathology. DIAGNOSES: Abdominal computed tomography demonstrated a contrast-enhanced 1.6 cm tumor-like mass in the distal CBD, leading to a presumptive diagnosis of malignancy. Laboratory findings showed an elevated γ-glutamyl transferase level, while other tumor markers were within normal ranges. INTERVENTIONS: Due to the high suspicion of malignancy, the patient underwent a Whipple procedure for both diagnostic and therapeutic purposes. OUTCOMES: Histological examination of the resected specimen surprisingly revealed adenomyomatosis of the CBD, confirming a benign diagnosis. The patient had an uneventful postoperative recovery and was discharged 19 days after surgery. LESSONS: This case highlights the diagnostic challenge posed by adenomyomatosis of the distal CBD, as its radiological appearance can closely resemble malignancy. Clinicians should be aware of this rare entity to avoid unnecessary aggressive surgical interventions.