Malignant transformation of osteosarcoma is a rare but severe complication that can occur in patients who have previously been treated for low-grade osteosarcoma. This complication is particularly concerning as it can lead to rapid deterioration of the patient’s condition and a significant decrease in their chances of survival. The risk factors for this complication are still poorly understood, but it is acknowledged that patients who have received incomplete or insufficient initial treatment are more likely to develop malignant transformation. We report the case of a 20-year-old man who developed malignant transformation of his low-grade osteosarcoma, treated with tumor resection without adjuvant therapy. The patient presented 2 years after conservative surgery with pulmonary metastases, as well as rare subcutaneous and cervical lymph node metastases. These latter metastases are particularly unusual and represent an additional challenge in patient management. This clinical case highlights the importance of multidisciplinary care in improving outcomes for patients with osteosarcoma. Regular surveillance and comprehensive evaluation of patients are essential for early detection of metastases and adapting management. The complexity of this rare condition requires an individualized approach, taking into account the specific characteristics of each patient. Further research is necessary to improve outcomes for patients with osteosarcoma.