Signet-ring cell carcinoma (SRCC) is rare in the biliary system. We report a case of SRCC coexisting with adenocarcinoma, arising in a choledochal cyst of the extrahepatic bile duct. The patient was a 52-year-old man, hospitalized for the investigation of jaundice and pruritus. Abdominal computed tomography and magnetic resonance cholangiopancreatography showed a huge choledochal cyst and distal common bile duct cancer. The patient underwent a pancreaticoduodenectomy with extended lymph node dissection. Histologic examination confirmed an SRCC coexisting with adenocarcinoma arising in a choledochal cyst. Postoperative chemotherapy had to be discontinued after only two cycles because the patient suffered serious side effects. Recurrence was detected in the bilioenteric anastomosis 4 months after surgery, and he died 6 months after surgery. To our knowledge, this represents the first case of SRCC arising in a choledochal cyst of the extrahepatic bile duct ever to be reported.