A four-year-old intact female domestic short-hair cat presented with chief complaint of polyuria, polydipsia, inappropriate urination and weight loss. After a thorough physical examination and bloodwork, the possibility of closed pyometra could not be ruled out, and ovariohysterectomy was performed. At surgery, a mass was noted in the left ovary and the uterus was seen to be dilated, with an area near the left ovary which was more dilated than the rest and filled with a thick, fluid-like substance. The right ovary was inapparent but a fluid-filled cystic structure was evident at the site. The tract was removed and submitted for histopathologic analysis. Granulosa cell tumor was found. Granulosa cell tumors are very rare in cats and this report will discuss a case of this disease.