Retroperitoneal sarcomas (RPS) are uncommon tumors associated with a poor prognosis. This is particularly true in case of high-grade sarcomas of specific histological subtypes, as demonstrated by the largest surveys of the last decade. Up to the present day, unfortunately there are no powerful tools available except for surgery. On the other hand, the resection rate of RPS is significantly increased over the last decades allowing to deliver the best treatment available. This paper reports on the case of a young patient who was incidentally diagnosed with a retroperitoneal mass. The patient underwent surgery in our department and the histological report showed a spindle cell sarcoma of high grade of malignancy with an incomplete muscular phenotype. The patient was discharged on the seventh postoperative day and he is still free of local and distant recurrence.