This report describes an extremely rare case of extrarenal Wilms' tumor in a 4 year-old girl who presented with abdominal mass and pain. Computed tomography (CT) scanning revealed a well-defined mass lesion measuring 10 cm on the right side of her lower abdomen and pelvic cavity. The mass was surgically removed. Histologically, the tumor showed a triphasic pattern, consisting of stromal, epithelial and blastemal components. Immunohistochemistry showed that the tumor was positive for cytokeratin, vimentin and CD99. The histopathological diagnosis was extrarenal Wilms' tumor arising in the retroperitoneum and inguinal canal. The patient was administered adjuvant chemoradiotherapy due to subsequent pulmonary and peritoneal metastases. Follow-up 4 years later showed that she was doing well, with no recurrence of the tumor.