We report here a rare case of adenocarcinoma arising from segmental adenomyomatosis of the gallbladder itself. CT images showed segmental annular wall thickening of the gallbladder body with regional lymph node enlargement, which could not exclude adenomyomatosis associated with malignancy. Therefore, we performed MRI and PET/CT for further evaluation. T2 weighted MRI revealed multiple tiny intramural cystic lesions in the focal wall thickening of the gallbladder, suggesting adenomyomatosis. However, PET/CT showed increased activity in the lesion of the gallbladder and regional lymph node, suggesting malignancy, which was pathologically confirmed as adenocarcinoma arising from adenomyomatosis.