An 88-year-old female with surgical history of acute appendicitis and right hemicolectomy 12 years ago was found to have a slow-growing lower abdominal mass on CT scan 3 years ago, subsequently a right pleural irregular-shaped nodule a year ago, and accumulation of fluid in the right pleural cavity when she was admitted to our hospital. According to the findings of jelly-like pleural fluid containing predominantly mucin and scanty atypical cells and enlarged low-attenuation mass around the right abdominal organs, she was diagnosed with pseudomyxoma peritonei (PMP) with pleural extension. Receiving palliative care for her anorexia and breathlessness, she died of respiratory failure caused by copious right pleural fluid 5 months after the diagnosis. An autopsy revealed low-grade mucinous neoplasm predominantly around the anastomotic intestines, with disseminated lesions on the surface of the diaphragm and pleural membrane, and no findings of organ invasion or lymph node metastasis. We concluded that our case of pseudomyxoma peritonei originated from low-grade appendiceal mucinous neoplasm with direct extension to the right pleural cavity through the right diaphragm.