Although soft tissue tension is an important factor in the clinical performance of reverse total shoulder arthroplasty (RTSA), this tension has not been quantified intraoperatively. Knowledge of the shoulder joint reaction forces during RTSA could facilitate the optimal placement of implant components to minimize the risk of both intra- and postoperative complications. We developed a strain gauge instrumented trial glenosphere to measure shoulder joint reaction forces during RTSA. The strain gauges and their connections were hermetically sealed against body fluids by biocompatible materials. All materials in direct body contact were biocompatible. In this study, we introduce the structure and calibration results of the instrumented prosthesis. We also demonstrate the practical use of the prosthesis on a cadaveric shoulder. The instrumented prosthesis showed mean measurement errors of approximately 3.4% for forces up to 400 N. A maximum joint reaction force of 132 N was observed during abduction in a single cadaver specimen. This sensor will be useful for quantifying soft tissue tension during RTSA surgery