A middle-aged man was being investigated for constipation. Abdominal radiographs incidentally revealed a large, densely calcified, rounded mass within the pelvic cavity. A CT scan was performed followed by surgical excision with a differential diagnosis of calcified hematoma and an enlarged calcified lymph nodal mass. Histopathological investigation revealed a primary mesenteric extraskeletal osteosarcoma. To the best of our knowledge, a primary extraskeletal osteosarcoma arising from the mesentery has not been described previously in the English literature. The radiological features and differential diagnosis are discussed.