Immune reconstitution inflammatory syndrome (IRIS) is particularly observed after the start of therapy for pathogenic antigens in patients infected with human immunodeficiency virus (HIV) and receiving highly active antiretroviral therapy (HAART). Although tuberculosis (TB)-associated IRIS is the most common form, its presentation as a primary feature of acute peritonitis is extraordinarily rare. We report a 43-year-old man diagnosed with acquired immunodeficiency syndrome and pulmonary TB coinfection. His symptoms, sputum quantity, and chest radiologic appearance improved markedly after 3 weeks of antituberculous therapy, and HAART was initiated on the fourth week. However, acute abdomen with peritoneal signs resulting from the established tuberculous peritonitis developed on the seventh day of HAART. His clinical symptoms resolved after maintenance of HAART and antituberculous regimens. Tuberculous peritonitis must be considered in the differential diagnosis of acute abdomen in HIV-infected patients on antiviral therapy, especially in patients with known underlying TB. Early recognition of IRIS is important when managing HIV-infected patients with opportunistic infections.