Desmoplastic small round cell tumor (DSRCT) is a rare, aggressive neoplasm that preferentially involves the abdominal and pelvic cavities in relatively young males. We present a rare case of DSRCT arising in the ovary of a 16-year-old girl. During surgery, a 15 cm-sized huge mass was noted in the right ovary and wide spreading of the tumor was identified in the left ovary, uterine wall, and omentum and bowel wall. Histological investigation showed nests of small round cells with round nuclei and scanty eosinophilic cytoplasm accompanied with dense desmoplastic stroma. The immunohistochemistry showed that the tumor coexpressed epithelial, mesenchymal, and neuronal markers. The tumor cells ultrastructurally showed poorly developed cell junctions and occasionally showed intracytoplasmic aggregates of intermediate filaments. Molecular analysis of the tumor revealed chromosomal translocation t(11:22)(p13;q12) associated with the EWS-WT1 fusion protein. DSRCT should be included in the differential diagnosis of ovarian neoplasms in young patients.