This report describes a case of newly recognized congenital M?llerian duct anomaly in a bitch. A fourteenyear-old, mixed breed bitch was presented with mild hematuria and mammary gland tumor. Complementaryexaminations were performed and abdominal ultrasonography revealed distended uterine horns and uterine tumor-likemass around the right ovarian area. Macroscopic examination showed the mass to be on the level of insertion of theright round ligament. The mass was cystic in nature, and had no communication with the uterine cavity. Histopathologically, the mass was identified as an accessory cavity lined by functional endometrium, which confirmedthe diagnosis of accessory and cavitated uterine mass.