Peritoneal carcinomatosis is a common metastatic manifestation of many organ-based malignancies, particularly carcinomas of the gastrointestinal tract and ovaries. Primary neoplasms of peritoneal and subperitoneal origin occur much less frequently than metastatic peritoneal involvement from a known or occult primary tumor; however, these rare primary lesions (peritoneal mesothelioma, papillary serous carcinoma, desmoplastic small round cell tumor, benign and malignant mesenchymal tumors, lymphoproliferative disorders) are often first detected at computed tomography (CT) and should be considered in the absence of a known or suspected organ-based malignancy. A precise diagnosis based on imaging findings alone is often not possible. Furthermore, distinguishing a benign from a malignant process and a primary from a metastatic process is also challenging. Nevertheless, CT features combined with the patient's relevant clinical and demographic data can help narrow the differential diagnosis for a peritoneum-based neoplasm in many cases. CT is useful not only for the detection, characterization, and staging of primary neoplasms of peritoneal and subperitoneal origin, but also for guiding biopsy for tissue diagnosis.