SummaryOsteosarcoma is a common primary bone malignancy most often involving the long bones and occurring in the second decade of life. Orbital involvement in this disease process is rare and is usually due to extension of the tumor from an adjacent sinus cavity. Orbital osteosarcoma arising from preexisting Paget disease is exceedingly rare. We report the case of a 78-year-old female patient with Paget disease of the forehead who presented with rapidly increasing bilateral proptosis and visual loss for several months. Examination showed bilateral aphakia, decreased motility, proptosis with resistance to retropulsion, and tumorous infiltration of the eyelids and periorbital soft tissue. Fundus examination showed marked bilateral scierai indentation without overlying serous retinal detachment. A computed tomographic (CT) evaluation of the orbits demonstrated extensive soft tissue involvement of the paranasal sinuses, nasal cavity, periorbital soft tissue, and orbits, with compression of the optic nerve and globe bilaterally. Biopsy of the periorbital soft tissue demonstrated osteosarcoma. Radiation therapy failed to stem the progression of the disease process, and the patient died ∼4 months later. Although osteosarcoma of the orbit associated with Paget disease is unusual, it should be considered in the differential diagnosis of older patients with a rapidly progressive orbital mass.