Well-differentiated osteosarcoma is extremely rare, there is no sex predominance and the mean age is in the third decade. The tumor has a strong predilection for the long bones of the extremities, especially the metaphysis but may also occur in the diaphysis. Radiologically, the lesion shows no distinctive features, often simulating fibrous dyplasia or desmoplastic fibroma. We report a case of well-differentiated osteosarcoma involving the rib of a 45-year-old female. This is a peculiar case of well differentiated osteosarcoma involving an unusual site and older patient. We also discuss histological differential diagnosis as well as clinical features of this rare disease.