Clinical symptoms at the time of presentation may include abdominal pain, abdominal mass, increased abdominal girth, distention of the abdomen, ascites (fluid in the abdomen), fever, weight loss, fatigue, anemia and digestive disturbances. Some patients complain of more non-specific symptoms for a number of months prior to a confirmed diagnosis. In a percentage of cases, peritoneal mesothelioma is found incidentally when the patient has sought help for another health problem such as gallbladder, hernia or pelvic mass.

Experienced doctors report that patients typically experience symptoms 6 months to 2 years before diagnosis. When the patient goes to the doctor, the patient, the family, and the doctor all usually think something else is wrong. Men often first show up with an inguinal hernia (a bulge in the groin) or an umbilical hernia (bulge around the belly button.) The first indication of a problem for some women comes during a pelvic examination when a tumor mass is discovered.

Late-stage peritoneal mesothelioma symptoms include bowel obstruction and increased tendency of the blood to clot. Blood tests show increased platelet count in half of peritoneal patients, although this is of little use in diagnosis because it can be caused by so many disorders. Anemia and low albumin levels are also found.