A 13-year old intact female poodle showed persistent signs of estrus. Ultrasonographically, a large cystic mass containing echogenic fluid with a thin wall was observed, and it was 8 cm in diameter. The cystic mass was located at the mesovarium between the left ovary and uterine horn, and it was identified as a giant parovarian cyst. A right ovarian mass was histologically diagnosed as a granulosa cell tumor. Persistent estrogen hormone secretion by the granulosa cell tumor was thought to cause a rapid increase in the size of the parovarian cyst. This rare cystic mass originated from the genital system and was differentiated from the uterus.