Peritoneal cancer is a rare disease that typically affects middle-aged women. Sclerosing mesenteritis can have a benign or malignant etiology. Although computed tomography (CT) scan and magnetic resonance imaging have been used to differentiate these two diseases, the findings are not always conclusive. Here, we report the case of an older woman who presented with acute abdominal pain. She was initially diagnosed with sclerosing mesenteritis, but the final diagnosis was peritoneal cancer. The initial treatment included antibiotics, non-steroidal anti-inflammatory drugs, and prednisolone. Tamoxifen was administered due to persistent symptoms, which were alleviated. However, the patient's cancer antigen 125 levels were elevated, and there were changes in the peritoneal CT findings. The patient was diagnosed with primary peritoneal cancer based on further investigation of the peritoneum using positron emission tomography-CT and a biopsy. This case report describes the diagnostic process regarding the differentiation between sclerosing mesenteritis and primary peritoneal cancer when the CT findings mimic those of sclerosing mesenteritis in general medicine.