Most existing transtibial prostheses are energetically passive. Their ankle joints are either rigid or rotatable in a limited range, and their feet are single-segment structures without toe joints. Amputees using these passive prostheses exhibit nonsymmetrical gait patterns, consume more metabolic energy, and walk at lower speeds compared with able-bodied individuals. In this paper, we design and construct a powered transtibial prosthesis with stiffness adaptable ankle and toe joints, which are driven by adapted series-elastic actuators, to improve the walking performance of the amputees. Mechanical models of both joints are built to help analyze joints' capabilities of adjusting stiffness. In actual control of the prosthesis, we utilize a linearized trajectory control method to adjust the stiffness of both joints. To evaluate the performance of the proposed prosthesis, experiments are carried out on an amputee with a unilateral transtibial amputation. Experimental results indicate that both ankle and toe angles of the proposed prosthesis are close to those of the sound limb, and the vertical ground reaction force of the prosthetic side is similar to that of the sound side. Compared with a commercial passive prosthesis, the proposed prosthesis can help the amputee obtain more natural and symmetrical walking gaits.