Accessory ovaries are rare entities defined as small portions of ovarian tissue situated near, and connected to, the normally placed ovary. Tumors arising in accessory ovaries are extremely rare. In particular, a fibroma arising from an accessory ovary has not been reported in the literature. We describe such a case with a fibroma developing in an accessory ovary. A 53-year-old multiparous woman presented with abdominal pain for 2 months. Pelvic computed tomography revealed 11.0x8.0x6.0 cm sized cystic mass with a thick septal wall in right adnexa. The preoperative diagnosis was a borderline ovarian tumor. Furthermore, a laparotomy showed a cystic mass connected to the right ovary by stalk. The bilateral eutopic ovaries were completely normal. Histologically, an accessory ovary was replaced by a fibroma accompanied by extensive cystic change.