Osteoblastoma is a rare neoplasm, accounting for only 1% of the primary bone neoplasms. We report a case of periosteal osteoblastoma, a rare subtype of osteoblastoma. To the best of our knowledge, only 30 cases of periosteal osteoblastoma appear in the English literature. Our case is that of a 41-year-old male with vague intermittent knee pain over a two-year period. Diagnostic imaging revealed an aggressive-appearing lesion in the posterior distal femur, which was initially considered to be a surface osteosarcoma. Roentgenographic, CT, MRI, and bone scan features are presented.