<p>Malignant transformation of adenomyosis is rare, with endometrioid carcinoma being the most common histological type. Herein, we report the first extremely aggressive case of undifferentiated carcinoma arising from adenomyosis. A 59-year-old Japanese woman was referred to our hospital with complaints of general fatigue, abdominal fullness, and loss of appetite. Contrast-enhanced computed tomography (CT) and integrated whole-body positron emission tomography/CT using ¹⁸F-fluorodeoxyglucose revealed multiple para-aortic and pelvic lymph node swellings. Contrast-enhanced magnetic resonance imaging revealed a 3-cm degenerated intramural uterine tumor. The patient underwent total hysterectomy, bilateral salpingo-oophorectomy, para-aortic and pelvic lymphadenectomy, and partial omentectomy for the treatment. Pathologically, the patient was diagnosed with undifferentiated carcinoma arising from adenomyosis. Despite adjuvant chemotherapy, the patient died of the disease 7 months after the initial surgery. Further investigation of similar cases may reveal the clinical features of this disease.</p>