This study presents an add-on external static balancing mechanism designed to improve the payload-to-mass ratio of serial robotic arms without requiring major structural modifications. The proposed mechanism consists of a five-bar linkage integrated with a preloaded tension spring and maintains the total potential energy—comprising gravitational and elastic components—as a constant throughout the arm’s range of motion. The mechanism was synthesized and kinematically analyzed to derive the necessary geometric constraints and spring stiffness required to achieve static equilibrium. Based on the specified link dimensions and spring parameters, the static balancing conditions were verified through analytical modeling. To validate the design, both numerical simulations and experimental testing using a 3D-printed prototype were conducted. The simulation and experiments validated the design, demonstrating an effective payload increase of 67.87% for the robotic arm. The proposed add-on device offers a simple, practical, and effective solution for enhancing the load-carrying performance of existing robotic arms.