Composite-allograft reverse shoulder arthroplasties are a surgical alternative in malignant tumor reconstructions following segmental bone resection of the proximal humerus. They allow reinsertion of the rotator cuff tendons and reliable restoration of humeral height, increasing prosthetic stability and mobility. Preoperative local tumor extension assessment must be thorough and a strict surgical technique must be followed to obtain satisfactory functional results. The indications for this procedure include a strictly intraosseous tumor of the proximal humerus corresponding to an S3-S4 A resection following criteria provided by the Musculoskeletal Tumor Society as described by Enneking and colleagues in 1990. Certain conditions must question the procedure such as extended S3-S4-S5 bone resections, partial sacrifice of the deltoid muscle, preoperative deltoid impairment, and glenohumeral arthrectomy. Here, we report technical details and indications of composite-allograft reverse shoulder arthroplasty in the context of tumor segmental resection of the proximal humerus, especially in the humeral preparation.