<p>Performing activities of daily living is one of the key challenges in the lives of transfemoral amputees. Although several research studies have been conducted on the design and development of transfemoral prostheses, some critical aspects, such as mechanical efficiency, mobility, mass, and compactness, are still challenges to address in this field. With this aim, in this study, we propose the design, development, and functional evaluation of a robotic knee prosthesis called Rnee. To efficiently generate mechanical power around the knee joint, a crank mechanism integrated with a motorized spring is utilized in the Rnee design. Thanks to utilizing the composite material in Rnee manufacturing, it is at least 10% lighter than one of the lightest robotic knee prostheses in the literature. The size of the developed Rnee is 7% smaller than the average dimension of the human knee. To demonstrate its mobility, a Li-ion battery pack is utilized to carry out 5000 steps of walking and 100 stair-ascending steps with a single charge, which would be sufficient for daily activities. The functional evaluations of Rnee are performed with an able-bodied participant. The results show that the gait cycle can be performed continuously using the Rnee. Despite the limitations of using the apparatus for the able-bodied participant, the kinematic results of Rnee resemble the natural gait cycle kinematics with a correlation coefficient of 82%. Additionally, the Rnee is able to perform the stair-climbing cycle successfully, generating 90% of the torque required for this task according to the mechanical and functional evaluations.</p>