Background and Clinical Significance: Malignant transformation of giant cell tumor of bone (GCTB) is a rare but clinically significant event, most commonly associated with radiotherapy, denosumab therapy, or recurrent disease. Secondary malignant transformation occurring in the absence of recognized risk factors is exceptionally uncommon. We report a rare case of high-grade sarcomatous transformation of proximal humeral GCTB after a prolonged latency period without prior radiotherapy, denosumab exposure, or documented recurrence; Case Presentation: A 27-year-old female initially presented with right shoulder pain and was diagnosed with proximal humeral GCTB. She underwent intralesional curettage and bone grafting, with histopathological confirmation of benign GCTB. Nine years later, she developed progressive shoulder pain, functional limitation, and systemic symptoms. Imaging demonstrated an aggressive lytic lesion with cortical destruction and soft-tissue extension involving the proximal humerus. Repeat curettage and histopathological evaluation revealed high-grade spindle cell sarcoma consistent with malignant transformation of GCTB. The patient received neoadjuvant chemotherapy followed by wide resection and endoprosthetic reconstruction of the proximal humerus, with additional adjuvant chemotherapy postoperatively. At two-year follow-up, she remained disease-free with excellent functional recovery and satisfactory quality of life; Conclusions: This case highlights the potential for delayed malignant transformation of GCTB even in the absence of established predisposing factors. Clinicians should maintain long-term vigilance in patients treated for GCTB, particularly when new pain, functional decline, or aggressive radiologic features develop years after initial treatment. Early recognition and multidisciplinary management are essential to optimize oncologic and functional outcomes.