A 5 year-old, intact female Yorkshire terrier was referred for dysuria and dyschezia. The radiographic and ultrasound examination showed a round shaped mass caudal to the urinary bladder that contained anechoic fluid within the thin walls. During surgery, the cyst was noted to be attached to the outer wall of the vagina, not connected to the vaginal lumen. Cystic fluid was removed and the cystic wall was resected. Then the remaining cystic wall was omentalized to prevent a recurrence. Histological examination confirmed that the cyst was of Wolffian duct origin. In this case, a large Gartner duct cyst causing urological problems was diagnosed and removed by surgical resection.