A 60-year-old man visited a nearby hospital with a 3-month history of back pain. Abdominal ultrasonography revealed a 9-cm mass in the peritoneal cavity, and the patient was referred to our hospital. On first examination, a movable mass was palpable in the abdomen. Blood examination revealed an elevated white blood cell count of 35,600/mm³. An abdominal computed tomography (CT) showed a mass measuring 9×6×7 cm in the mesentery, but no abnormalities were detected in the bone marrow biopsy. Hence, a diagnosis of reactive leukocytosis associated with an intraperitoneal tumor was made. Open surgical biopsy led to a diagnosis of inflammatory myofibroblastic tumor (IMT). Judging that there was no effective treatment other than resection, we surgically removed the tumor. The final histopathological diagnosis was inflammatory malignant fibrous histiocytoma (MFH). Documented cases of mesenteric MFH are rare. Herein, we report a case of MFH that was originally identified as IMT by surgical biopsy, but finally diagnosed as MFH by postoperative reassessment, and we also discuss the relevant literature.