Osteosarcomas are a heterogeneous group of tumors with diverse anatomical, clinical, and progressional characteristics. Parosteal osteosarcoma, or juxtacortical osteosarcoma, is a rare form of osteosarcoma that develops at the bone surface, but has a better prognosis than other conventional osteosarcomas. We report the observation of a 22-year-old female patient whose initial presentation was an enormous tumefaction of the knee that had been progressing for 10 years. The biopsy concluded in PO of the lower third of the femur. Staging was negative. The tumor had reached an enormous size and required amputation of the left lower extremity. A custom external prosthesis was manufactured to get her back to walking. Eight years after surgery, no local recurrence or metastasis has been detected. Parosteal osteosarcoma is a rare form of osteosarcoma with very slow progression (in spite of the particularly dramatic presentation in our observation), with an excellent prognosis and very rare metastasis.