Bio-inspired flapping propulsion offers remarkable hydrodynamic efficiency; however, replicating the complex active hydro-elastic behaviors of biological swimmers, such as penguins, remains a significant robotic challenge. Existing rigid or passively flexible foils often fail to exploit the full potential of real-time stiffness modulation. To address this, this paper presents a novel bio-inspired morphing hydrofoil featuring a tendon-driven articulated structure that mimics the penguin’s carpal joint. We establish a comprehensive theoretical framework coupling multi-body kinematics with continuum stiffness analysis to guide the design. A specialized core-to-skin multi-material fabrication process is developed, integrating a rigid transmission system within a gradient-stiffness silicone body. Experimental characterization validates the mechanism’s ability to actively decouple structural stiffness from geometric configuration, achieving a tunable stiffness range. Furthermore, preliminary unsteady hydrodynamic simulations demonstrate that the active morphing mode generates superior thrust profiles compared to fixed-stiffness baselines. This work provides a compact and robust solution for next-generation variable-stiffness underwater propulsion.