Giant cell tumor (GCT) is a rare complication of Paget disease of bone. It usually occurs in the skull or pelvic bones of patients with long-standing polyostotic disease. This report describes a 62-year-old patient who presented with monostotic Paget disease of the distal femur complicated by GCT. He had a 2-year history of discomfort and pain in his left knee. Conventional plain films and MRI demonstrated the characteristic bone changes of Paget disease and an associated lytic lesion involving the epiphyseal and metaphyseal regions of the distal femur. A diagnostic curettage showed the characteristic histopathologic features of Paget disease and GCT. There was no evidence of malignancy. The clinicopathologic features of this rare lesion are described and correlated with a review of the literature.