A case of mucinous tumor of the gallbladder with a separate nodule of anaplastic carcinoma is reported. The patient was an 83-year-old Japanese man who underwent cholecystectomy under the preoperative diagnosis of a mucus-producing gallbladder tumor. A mucinous tumor was found in the neck and distal body of the gallbladder, associated with a separate nodule in the fundus. The latter nodule was initially diagnosed as a benign xanthogranulomatous lesion. However, the immunohistochemical study revealed that the atypical cells in the superficial part of the nodule were positive for cytokeratin and epithelial membrane antigen, confirming the diagnosis of anaplastic carcinoma. Although the occurrence of mural nodules in mucinous cystic tumors of the ovary and pancreas is well reported, to our knowledge, this is the first report on the occurrence of a mucinous tumor with a nodule of anaplastic carcinoma in the gallbladder.