Osteosarcoma of the proximal humerus presents a significant surgical challenge requiring wide excision and reconstruction. Megaprosthetic reconstruction is the preferred surgical treatment, though long-term complications, such as implant failure, may necessitate revision surgery. In this study, we present a case of a 27-year-old female who underwent a successful revision surgery for a previously reconstructed proximal humerus (with a hemi-mega prosthesis) using Reverse Shoulder Arthroplasty (RSA) with mega prosthesis. Clinical and radiographic evaluation had revealed superior migration of the previous implant, requiring revision surgery to relieve her symptoms. The procedure involved the removal of the previous hemi-mega prosthesis, preparing the humeral shaft, and implantation of a reverse shoulder system. Postoperatively, the patient experienced significant pain relief and improved shoulder function. This case highlights the effectiveness of RSA involving a mega prosthesis in complex revision surgeries, demonstrating its usefulness in restoring stability, mobility, and function in patients with failed primary reconstructions.