Most cases of spontaneous bacterial peritonitis (SBP) in association with nephrotic syndrome are children. The complication of SBP in adults with nephrotic syndrome is extremely rare. Herein, we report a 25-year-old man with nephrotic syndrome and chronic renal failure who suffered from SBP. Citrobacter freundii was isolated from ascites. Irreversible deterioration of renal function followed the development of SBP, though the peritonitis was cured with antibiotic treatment. This case suggests that SBP is a rare, but serious complication of adult nephrotic syndrome with ascites.