A 55-year-old woman with painful splenomegaly underwent surgical exploration and resection of the spleen. Pathologic examination revealed a neoplasm of the spleen exhibiting malignant stromal and epithelial components with müllerian features. In the absence of a recognized neoplasm elsewhere, the carcinosarcoma most likely arose as a primary splenic tumor and may represent an extrauterine malignant mixed müllerian tumor arising in the spleen.