Robots designed for human interactions require not only precision but also adaptability and safety during physical contact. Traditional rigid robots perform well in structured industrial environments but are limited in collaborative applications because of their inherent lack of compliance. Soft robots, by contrast, offer intrinsic safety and adaptability but struggle with accurate modeling and controllability. To address these limitations, this paper proposes a novel design strategy for robotic mechanisms that integrates rigid and soft components to exploit the advantages of both principles. This strategy is demonstrated through a hybrid slider–crank mechanism incorporating a soft, variable stiffness connecting rod into the conventional rigid framework. The soft connecting rod, constructed as a pneumatic pressure–responsive elastomer composite, allows active stiffness modulation: elevated pressure increases rigidity for accurate force transmission, whereas reduced pressure enhances compliance for safer interaction. A theoretical model of the mechanism is developed to describe stiffness variation, dynamic force transmission under different pressurization levels. The design and fabrication process of the soft connecting rod is presented, followed by indentation tests to characterize pressure-dependent stiffness and dynamic experiments to evaluate force transmission in the hybrid mechanism. Experimental results confirm that pneumatic pressurization effectively tunes the stiffness of the connecting rod, leading to controllable changes in dynamic response and output force. This work demonstrates the feasibility of embedding soft variable-stiffness elements into core structural components of classical mechanisms, paving the way for collaborative robots that combine precision, adaptability, and safety.