Florid reactive periostitis ossificans is a well-known benign lesion classically described in hands and feet which histopathological features can lead to a misdiagnosis of osteosarcoma. To the best of our knowledge, there is only one previous report of this lesion in a long bone. In this study we report a case of florid reactive periostitis ossificans located in the distal metaphysis of the left femur that histologically mimicked an osteosarcoma and discuss the differential diagnosis between these two entities to warn about a diagnostic pitfall.