A 48-year-old woman presented with a hard, painless mass in the left lower quadrant and no other significant complaint. Laboratory tests, including those for tumor markers, showed no significant abnormal value. CT showed an extraperitoneal mixed solid and cystic mass in the anterior abdominal wall, located anterocranial to and in close contact with the bladder. Percutaneous biopsy findings were compatible with adenocarcinoma of unknown origin. PET/CT showed increased FDG uptake on the mass and no evidence of disease at other sites. The patient underwent tumor resection, and the pathological diagnosis was moderately differentiated urachal adenocarcinoma.