The primary retroperitoneal serous adenocarcinoma (PRSAC) is a rare malignant tumor of the retroperitoneum. It shares the same pathological and biological behavior with ovarian serous carcinoma. Most of the cases develop as peritoneal adenocarcinoma and rarely occur in the retroperitoneum. It is reported as serous surface papillary carcinoma of the peritoneum and extraovarian peritoneal serous papillary carcinoma. We present a case of PRSAC in a 60-year-old woman. Only 11 cases of PRSAC have been reported from 1983 to 2019. Histopathological features with immunohistochemical expressions are important to diagnose PRSAC. The outcome and survival mainly depend on the possibility of surgical resection. Molecular genetics of PRSAC should also be studied in relation with its ovarian counterpart.