A primary serous papillary cystadenocarcinoma of the rectovaginal septum was diagnosed in a 35-year-old woman. There were invasive microfoci in peritoneum and lymph node micrometastasis as well as a small superficial non invasive tumor focus in the left ovary. Diagnosis was based on morphological, immunohistochemical and ultrastructural features of tumor tissue. The patient had surgery and post-operative chemotherapy. Progression of peritoneal disease was found six months after surgery. In this very rare location of serous carcinoma, tumor might have developed from peritoneal inclusions in the rectovaginal septum.