Giant cell tumor of bone (GCTB) is a primary bone neoplasm composed of mononuclear cells proliferating among scattered macrophages and large, osteoclast-like giant cells. Secondary malignant GCTB is a sarcoma arising at a site where a GCTB has been previously documented. This case report is about a 32- year-old female who presented with right knee pain of one year duration. The patient had a history of bone tumor, at the same site, six years ago. Radiographs showed a mass in the epimetaphy seal, extending into the tibiofemoral and patellofemoral junction. Such findings were consistent with a recurrent lesion. Following microscopic investigation, spindle- cell with atypical mitoses and nuclear pleomorphism, admixed with conventional GCTB, were identified. Sarcomatous changes in malignant GCT develop many years, or even decades, following the primary lesion’s removal. Radiologic features are represented by large lesions with cortical destruction and prominent invasion into the soft tissue. Microscopically, the conventional GCTB is adjacent to, or intermixed with, the sarcomatous area. Based on such findings, we conclude this case is a secondary malignant GCTB. © 2020 by Nova Science Publishers, Inc. All rights reserved.