Inadequate soft tissue tension around the shoulder joint is the known cause of postoperative complications following reverse total shoulder arthroplasty (RTSA). However, this tension is not currently quantified intraoperatively. Understanding soft tissue tension can help surgeons choose the appropriate implant size and alignment, potentially reducing the risk of complications. To address this, we developed a force sensor that measures joint contact force in the shoulder to estimate soft tissue tension. The force sensor includes strain gauges and a printed circuit board for analog-to-digital conversion. It is waterproof and can be sterilized, making it suitable for use during surgery. In this study, we present the force sensor’s design, results from the finite element analysis of output strain values of a sensing element, and calibration results. The results of the strain analysis showed a strong correlation between the output strain and the applied force, with correlation coefficients above 0.999 across all three axes (X, Y, and Z). As a result of the calibration, the correlation coefficients were 0.8284 on the X-axis, 0.8078 on the Y-axis, and 0.9413 on the Z-axis. The maximum errors were 8.5 N on the X-axis, −13 N on the Y-axis, and −6.1 N on the Z-axis. Although the errors are large, this force sensor is useful for quantifying soft tension tissue during RTSA.