This paper presents a hierarchical control strategy for a robotic transtibial prosthesis to realize smooth locomotion transitions between level ground and stairs. The high level controller identifies current terrain with a fuzzy logic based method and decides the corresponding parameters for lower level controllers. The middle level controller detects different gait phases of one gait cycle on a specific terrain and decides which control method to be used for the current phase. Based on the recognized terrain and gait phase, the low level controller performs damper control for the stance phase and angle control for the swing phase. To evaluate the effectiveness of the proposed control method, we design and construct a robotic transtibial prosthesis prototype. Experimental results of a transtibial amputee subject show improved gait symmetry on level ground and stairs with the proposed control strategy. A 12-s long trial that includes different terrains and terrain transitions indicates that the proposed method can realize smooth locomotion transitions between level ground and stairs.