Herein is reported a case of mucin-producing carcinoma of the gallbladder in a 55-year-old Japanese woman. Although the patient's status and laboratory data initially suggested biliary pancreatitis due to gallstone, radiography and endoscopy confirmed the presence of pancreaticobiliary maljunction and a gallbladder tumor with excessive mucin, in which the duodenal papilla and the common bile duct were impacted. Following surgery, the gallbladder tumor was histopathologically diagnosed as a mixed endocrine–exocrine carcinoma. The carcinoma predominantly consisted of papillary, but also contained some tubular adenocarcinomatous components. Additionally, small foci of small cell and large cell neuroendocrine carcinomatous components were observed. There was no evidence of lymph node metastasis, distant metastasis, or direct invasion outside the gallbladder. Thus, the final classification of pT2N0M0 stage II was given to this lesion, according to the Union Internationale Contre le Cancer guidelines. The postoperative course was uneventful, and the carcinoma had not recurred in the absence of chemoradiotherapy for a period of 20 months. Mucin-producing gallbladder carcinoma is a rare clinical condition that can occur in patients with pancreaticobiliary maljunction. Detailed investigation of this condition is important to develop and refine effective therapeutic strategies.