Primary serous papillary carcinoma of the peritoneum is a rare tumor, histologically similar to primary ovarian carcinoma. Pelvic CT and MRI are helpful to diagnose primary carcinoma of the peritoneum. We present a case of primary serous carcinoma of the peritoneum mimicking pelvic actinomycosis in a 59-year-old woman. Pelvic CT and MRI suggested pelvic actinomycosis. Exploratory laparotomy was performed to remove the mass and the diagnosis was confirmed by pathology. A subtotal hysterectomy, left salpingo-oophorectomy and omentectomy were performed. Histopathology examination revealed peritoneal carcinomatosis and primary serous carcinoma of the peritoneum. Pelvic CT and MRI were limited in their ability to differentiate inflammation, such as actinomycosis, from primary carcinoma of the peritoneum.