Primary malignant giant cell tumor of bone (PMGCTB) refers to a high-grade sarcoma arising from adjacent to benign giant cell tumor located in bone tissue. This type of sarcoma is rare and accounts for approximately 7% of all giant cell tumors. This report shares our experience of diagnosing a rare case. A 53-year-old man presented with a mass on the upper end of his tibia that appeared 6 years before he was admitted to the hospital and had no previous history of radiation therapy or surgery. A radiograph showed an aggressive bone tumor at the metadiaphysis with a suspicious femorotibial joint involvement. Histopathology showed mononuclear cell proliferation, multinucleated giant cells with abundant nuclei, identical with mononuclear cells in the stroma, and a high-grade sarcoma area juxtaposition to the benign giant cell tumor. PMGCTBs arise de novo and are extremely rare. From these findings, we concluded this case as PMGCTB. Malignant giant cell tumor of bone (MGCTB) prognosis is poor with a 5-year survival of 50%, and PMGCTB has a better prognosis than secondary MGCTB. © 2020 by Nova Science Publishers, Inc. All rights reserved.