Dienogest is a novel synthesized progestin used for treatment of endometriosis. This is the first case report describing a therapeutic effect of dienogest on a gynecologic malignancy. The patient was a 44-year-old woman with advanced adenosarcoma arising from the endometriosis in the rectovaginal space and infiltrating the left pelvic wall, left ureter, rectum and vagina. The residual tumor after tumor debulking surgery was resistant to both chemotherapy and radiotherapy. Dienogest was used as a substitute for medroxyprogesterone acetate because of the presence of deep vein thrombosis. Based on the RECIST criteria, partial response was obtained with oral dienogest therapy at six months and the serum CA125 level also decreased from 70 U/ml to 16 U/ml. The tumor remained stable up to 21 months. Thromboembolism or other adverse effects did not occur during the dienogest therapy. Dienogest may be useful for the treatment of adenosarcoma arising from endometriosis.