Background: Osteoarticular allografts and allograft-prosthesis composites have decreased some complications but have introduced others, prompting development of implants that combine a reverse total shoulder arthroplasty (RTSA) prosthesis with a modular endoprosthetic reconstruction system. Methods: Data were collected prospectively from two institutions for all patients with massive proximal humeral bone loss who had reconstruction with RTSA using a single modular endoprosthetic system. The primary outcome measure was failure of the reconstruction, defined as need for revision surgery or radiographic evidence of component failure. Preoperative and postoperative functional outcomes and patient-reported outcomes, determined through preoperative and postoperative Quick Disabilities of Arm, Shoulder and Hand (quickDASH) scores, American Shoulder and Elbow Surgeons Standardized Shoulder Assessment Form (ASES) scores, and visual analog scores (VAS), were compared. Results: Seven female and five male patients met inclusion criteria. The most common reason for proximal bone loss was failure of a previous arthroplasty procedure (six patients). Average forward flexion and internal rotation improved significantly, as did the average quickDASH, ASES and VAS scores. Failure of humeral component fixation occurred in three patients. Conclusions: Early results show that a RTSA prosthesis using a modular endoprosthetic reconstruction system is a viable option for massive proximal humeral bone loss. Further studies are needed to investigate longevity and longer-term outcomes. Level of Evidence: Level IV.