In August 2022, a 59-year-old female noted a mass in her umbilicus and sought evaluation at Toyokawa City Hospital. Abdominal computed tomography(CT)scan revealed a 1.6 cm mass in the umbilical region, ascites in the pelvis, and increased absorption in the omentum. Peritoneal dissemination of the carcinoma and Sister Mary Joseph's nodule due to an unknown primary tumor were suspected because no abnormalities were detected during upper and lower gastrointestinal endoscopy. She underwent an umbilical lumpectomy and diagnostic laparoscopy to establish a definitive diagnosis. The surgical findings included numerous white nodules throughout the abdominal cavity. The umbilical mass and omental white nodules were resected. A final diagnosis of epithelial peritoneal mesothelioma was made based on the histopathologic examination. In general, peritoneal mesothelioma has a poor prognosis, and early treatment is essential; however, making a timely definitive diagnosis is difficult. Peritoneal mesothelioma should be included in the differential diagnosis for a patient with unexplained ascites and abdominal pain. Diagnostic laparoscopy and biopsy will facilitate the establishment of a definitive diagnosis.