Alveolar soft part sarcoma (ASPS) is a rare type of neoplasm, typically occurring in adolescents and young adults. Because of the rarity of this disease, there is no standard treatment plan. Chemotherapy and radiotherapy are not effective in this type of malignancy. Surgical excision is considered to be the treatment of choice. We report a case of a young woman with a painless mass in her left lower abdomen. Ultrasonography and CT scan revealed a large (21 cm) hard tumor occupying the left retroperitoneal space, which was surgically excised in our surgery department. The pathological diagnosis was ASPS.