This paper presents the design and control of a pneumatically actuated transtibial prosthesis, which utilizes a pneumatic cylinder-type actuator to power the prosthetic anlde joint to support the user's locomotion. The pneumatic actuator has multiple advantages over the traditional electric motor, such as light weight, low cost, and high power-to-weight ratio. The objective of this work is to develop a compact and lightweight transtibial prosthesis, leveraging the multiple advantages provided by this highly competitive actuator. In this paper, the design details of the prosthesis are described, including the determination of performance specifications, the layout of the actuation mechanism, and the calculation of the torque capacity. Through the authors' design calculation, the prosthesis is able to provide sufficient range of motion and torque capacity to support the locomotion of a 75 Kg individual. The controller design is also described, including the underlying biomechanical analysis and the formulation of the finite-state impedance controller. The testing results of human subject indicate that the prosthesis is able to generate a natural walking gait and sufficient power output for its amputee user.