Few studies report on tissue morphology in recurrence of yolk sac tumor. The case of the recurrence of a yolk sac tumor as a spindle cell sarcoma of the abdominal wall is presented. A 27-year-old woman was referred to our hospital due to suspicion of an ovarian tumor. Right salpingo-oophorectomy, partial omentectomy, and extirpation of disseminated foci as fertility-preserving surgery was done since the intraoperative pathological diagnosis was yolk sac tumor. Final pathological examination showed a germ cell tumor of which yolk sac tumor formed the major component including a small area that appeared to be immature nerve tissue. Although residual tumor was not less than 1 cm, clinical complete remission was reached after the sixth course of BEP regimen. However, the recurrence of a yolk sac tumor as an unclassified spindle cell sarcoma of the abdominal wall was found about two years after the initial surgery. Thereafter, the patient expired due to progression of the intraperitoneal disseminated lesions. The mesenchyme- like component of the yolk sac tumor is characterized by spindle cells originating from epithelial elements, and is likely to give rise to a chemoresistant, diversely differentiated sarcoma. This report suggests that the sarcoma reported in the case here also arose when spindle cells of the mesenchyme-like component underwent sarcomatous change during or after chemotherapy, subsequently relapsed as a chemoresistant tumor, and metastasized.