Nowadays, many people suffer from physical disabilities caused by illness or accidents. The problems in the lower limb are one of the main that interrupt the activities of daily life of these people. To assist them in the activities of daily living, many support devices for upper and lower limb movement have been developed. In this paper is proposed a new simple and low-cost pneumatic robotic mechanism for lower limb rehabilitation. We employ a pneumatic actuator so that it is possible to obtain safety for the operation and the control of the force by the appropriate regulation of the pressures in the pneumatic cylinder chambers of the robot for rehabilitation. This work deals with the development of the robot for rehabilitation from a methodology of integration of mathematical modeling with the phases of the design process. It is concluded that the results obtained are a modular design that can be improved for multiple degrees of freedom and adapted also for rehabilitation of upper limbs.