A solitary peritoneal metastasis from a poorly differentiated adenocarcinoma of unknown origin in a 38-year-old female is described. The patient presented with a left ovarian tumor and elevated cancer antigen 125. She underwent complete excision of tumor and received six courses of postoperatively adjuvant cisplatin-based combination plus radiation therapy simultaneously. Second-look operation was done later and complete surgicopathologic remission was demonstrated. Two further courses of consolidation chemotherapy were given. She has been followed up for 25 months, and is living and well. We suggest that the combination of aggressive surgery, cisplatin-based chemotherapy and an open-field technique of abdominopelvic irradiation (concurrent chemoradiation) may be an appropriate approach to manage solitary peritoneal metastases from poorly differentiated adenocarcinoma of unknown origin.