A 53-year-old Japanese female was admitted to our hospital for examination and treatment of an abdominal mass lesion. Contrast-enhanced abdominal computed tomography (CT) showed a well-circumscribed mass that measured approximately 5 cm. Although the tumor was located near the neck of the gallbladder, it seemed to be pedunculated from the liver parenchyma. From the enhancement pattern and morphological characteristics of the contrast CT images, the nodule was preoperatively diagnosed as hepatocellular carcinoma. The patient underwent a laparotomy, and it was intraoperatively revealed that the tumor existed solely in the neck of the gallbladder without serosal invasion. Whole-layer cholecystectomy with lymph node dissection was therefore performed. A pathological examination revealed that the papillary tumor filled up the neck side lumen of the segmental type of adenomyomatosis (ADM). The tumor was histologically diagnosed as papillary adenocarcinoma. Cases of papillary adenocarcinoma arising on the neck side lumen of the segmental type of ADM are extremely rare. Furthermore, no such case of gallbladder cancer which mimicked hepatocellular carcinoma has previously been reported.