Primary retroperitoneal leiomyoma is a rare and unusual differential diagnosis in the gynecological practice. The pathogenesis of retroperitoneal leiomyoma remains obscure. A 49-year-old nulligravida woman admitted to our hospital with complaints of nocturia, lower abdominal pain and a large abdominopelvic mass. The diagnostic workup included transabdominal ultrasonography, contrast computerized tomography and magnetic resonance imaging that revealed a 15-cm homogeneous pelvic mass with multiple septations. Retroperitoneal leiomyoma was suspected on imaging studies and was confirmed by tumor excision and histopathological examination.