Culture-negative peritonitis accounts for up to 20% of all peritoneal dialysis-related peritonitis, the causes of which include culture-associated technical errors, prior use of antibiotics, infection caused by certain unusual or fastidious microorganisms, the development of abdominal or retroperitoneal organ inflammation, and the presence of malignancies. Here, we report a patient with end-stage renal disease receiving peritoneal dialysis who presented with culture-negative peritonitis and ultrafiltration failure caused by the rare pseudomyxoma peritonei. For cases of culture-negative peritonitis such as this, early imaging studies may help recognize intraperitoneal/retroperitoneal visceral inflammation and malignant conditions.