Peritoneal disease can manifest at computed tomography (CT) as fluid accumulation within the peritoneal cavity (ascites) or soft-tissue infiltration of the various peritoneal ligaments and mesenteries. Beyond the commonly encountered cases of typical ascites and peritonitis, there is a wide spectrum of uncommon nonneoplastic conditions that may involve the peritoneal and subperitoneal spaces. For example, systemic or organ-based diseases that occasionally involve the peritoneum include eosinophilic gastroenteritis, amyloidosis, extramedullary hematopoiesis, Erdheim-Chester disease, sarcoidosis, and mesenteric cavitary lymph node syndrome. Tumorlike conditions that may affect the peritoneum include aggressive fibromatosis (desmoid), inflammatory pseudotumor, retractile mesenteritis, and Castleman disease. Atypical peritoneal infections include tuberculosis, actinomycosis, echinococcosis, Whipple disease, and mesenteric adenitis. Conditions involving the subperitoneal fat include epiploic appendagitis, mesenteric panniculitis, and segmental omental infarction, all of which have characteristic CT findings. CT is an excellent imaging modality for detection and characterization of peritoneal involvement from these unusual diseases.