We present an extremely rare case of ovarian serous cystadenoma with mural nodules of rhabdomyoma. The patient, a 48-year-old woman, was admitted to our hospital with left lower abdominal pain and vaginal bleeding. A unilocular cystic tumor, measuring 13 x 10 x 10 cm, was found in her left ovary and was removed. The tumor contained clear serous fluid, approximately 600 mL, and 2 mural nodules, up to 7.5 x 5.5 x 4.5 cm. The internal cystic wall was thin for the most part and lined with ciliated cuboidal epithelium without any malignant feature. The mural part was composed of mainly more mature muscle fibers with easily discernible cross-striations, set in abundant myxoid to fibromyxoid stroma, similar to clinical and microscopic manifestations of genital rhabdomyomas reported in other sites. Because extracardiac rhabdomyoma has never been described occurring in the ovary, especially arising in serous cystadenoma, to our knowledge, this is the first case reported in the English literature.