Malignant peritoneal mesothelioma is a rare malignancy of the serosal membranes that is difficult to diagnose and carries a poor prognosis. Progression of presenting symptoms is slow and nonspecific. Diffuse peritoneal seeding by the neoplasm can present silently so that physical abnormalities may not be detected until the disease is far advanced. Standard laboratory and radiographic studies can also be inconclusive. We report a case of an adolescent female whose clinical presentation and initial response to management were consistent with inflammatory bowel disease (IBD) but who was ultimately diagnosed with malignant peritoneal mesothelioma.