Ovarian tumors in cows are uncommon and most are granulosa cell tumors. Bovine ovarian fibromas are extremely rare and tend to occur in older animals. This paper reports the case of a large ovarian mass found in a primiparous dairy cow which had shown no sign of cycling after calving. The mass continued to increase in size as detected during six-month serial observations by ultrasonography. An ovariectomy was subsequently performed and the growth was diagnosed as an ovarian fibroma by gross and microscopic examination. This case is the first report of an ovarian fibroma observed in a living dairy cow.