In August 2010, a 73-year-old woman with rheumatoid arthritis receiving etanercept (ETN) therapy for two years, developed high-fever and abdominal fullness. Though she had not been exposed to tuberculosis, isoniazid prophylaxis was administrated. Antibiotics were not effective. CT images revealed the massive ascites and peritonitis, and Ga scintigraphy demonstrated notable uptake in the peritoneum. Ascites analysis showed an elevated adenosine deaminase activity value (104.9 IU/l) without malignant cells. Moreover, PCR and culture for Mycobacterium tuberculosis were positive. Finally, a diagnosis of tuberculous peritonitis was established. After initiating a standard anti-tuberculosis regimen with four drugs, her clinical condition ameliorated and ascites promptly regressed. Although the tuberculous peritonitis during ETN therapy is rare, this report emphasized the importance of initial suspicion of tuberculosis in these patients with tumor necrosis factor inhibitors such as ETN.