We report the case of a pelvic malignant peripheral nerve sheath tumor mimicking an adnexal mass. A 59-year-old postmenopausal woman presented with a 3-month history of diffuse abdominal bloating and urinary frequency. Laboratory tests revealed an increased CA 125. Radiologic evaluation depicted a large, heterogeneous solid mass located right to the uterus, pushing it to the left. After a multidisciplinary board discussion, the diagnosis of a right adnexal lesion was assumed, and the patient was referred to surgery. The final diagnosis was only achieved after pathology examination, which prove to be a malignant peripheral nerve sheath tumor. This paper highlights some clinical, radiologic and pathological features of malignant peripheral nerve sheath tumors, a rare entity that should be considered as a differential in patients presenting with pelvic tumors of uncertain origin.