Urachal malignancy is a rare and represents less than one percent of bladder neoplasms. Urachal mucinous neoplasms are still rare and include both villous adenomas and invasive adenoma carcinomas and they have insidious course and variable clinical presentation. We present a case report of 58 year old male who presented with lower abdominal mass since 6-8 months with no lower urinary tract symptoms, significant history of loss of weight and appetite. On examination the infra-umbilical midline mobile mass of 10×10 cms was palpable. CT showed lobulated cystic mass with internal septations and calcifications superior to bladder with indentation and without any local or distant metastasis. Mass was excised in toto with partial/ segmental cystectomy. Histology revealed mucinous cystadenoma of undetermined malignant potential, rarest histological variety and difficult to diagnose on basis of mucin histochemistry and immunohistology. Two-year follow-up did not show any local tumor recurrence.