Primary papillary serous carcinoma of the peritoneum is a rare primary tumor involving the peritoneum. Histologically, it is indistinguishable from serous ovarian papillary carcinoma, although it either spares the o-varies or only microscopically involves their surface. The characteristic features of this tumor are extensive peritoneal and omental masses or implants with psammomatous calcification, and ascites. In addition, it can occur focally in the pelvic peritoneum. We report the CT findings, with histopathologic correlation of primary papillary serous carcinoma of the peritoneum occurring in the upper mesorectum.