We report an extremely rare case of adult extrarenal Wilms' tumor (WT) in a 52-year-old woman who presented with fever and abdominal distension. Computed tomography revealed a well-defined mass lesion measuring 15.0 cm in the right retroperitoneum and that was in contact with the right kidney. The mass and kidney were surgically removed. Grossly, the mass was well-defined, measuring 16.3 x 11.0 x 9.8 cm, and appearing grayish-white in color. The border between the mass and the kidney was well-defined. Histologically, the tumor showed a triphasic pattern consisting of stromal, epithelial and blastemal components. The stromal component was predominant in the tumor and consisted both of spindle cells and smooth muscle cells. The epithelial component showed a mature glandular structure. Immunohistochemically, the stromal component was positive for vimentin, smooth muscle actin and desmin. The blastemal component was positive for vimentin, while the epithelial component was positive for cytokeratin (CK) 18, CK7 and vimentin. WT-1 was negative in the all three components, and the Ki-67 proliferation index was low. The postoperative histopathological diagnosis indicated extrarenal WT arising in the retroperitoneum. Although not treated by either chemotherapy or radiation therapy, she was free from disease recurrence for 30 months after surgery. To the best of our knowledge, this report is only the fourth case of adult extrarenal WT arising in the retroperitoneum. Furthermore, the present case showed predominant smooth muscle differentiation and a mature glandular structure, mimicking a mixed epithelial and stromal tumor.