A 32-year-old female presented with a large tumoral mass in the left upper quadrant (ULQ) of the abdomen with abdominal and retroperitoneal components, infiltrating the spleen and adhered to pancreatic tail, colon, left kidney, and stomach that could be completely resected along with the spleen. Histopathologic study including immunohistochemical profile was coincident with results obtained at Barnes-Jewish Hospital of Washington University Medical Center, USA. This study showed areas of granulosa tumor cells as well as thecal features of fusiform cells with eosinophilic cytoplasm and tubular formations corresponding to Sertoli cells. According with embryologic development, the specialized stroma of the genital crest is induced to proliferase after primordial germ cells infiltrate the crest; in this way, tumoral growth can occur at any site along this migration. The predominating localization of this unique ectopic tumor coincides with the descending route of the ovaries or its components. This case, included in unclassified forms of stromal sex cord tumors, is considered a monodermal malignant teratoma. Is was the second tumor of retroperitoneal localization and the eleventh ectopic, according to the world literature reviewed.