Mycobacterium avium complex (MAC) frequently disseminates in AIDS patients, where the gastrointestinal tract is a major target organ. While ascites in AIDS patients is common, peritonitis secondary to MAC is rare. We describe the first case of MAC peritonitis in an AIDS patient without underlying cirrhosis, portal hypertension, chylous ascites or peritoneal dialysis. This case highlights the need to be aware of atypical presentations of MAC disease in AIDS patients with a history of disseminated MAC, even those who compliantly take highly active antiretroviral therapy.