Rehabilitation robotics is a tool to support rehabilitation therapist that reduces the patient's recovery time and improve the exercises repeatability.Arm rehabilitation devices have been developed to perform the exercises in the horizontal plane; however, most devices do not consider arm pronosupination.This paper presents the design of a pronosupinator that can be mounted to the end effector of some existing devices.The pronosupinator consists of a sling and a cylinder that allows forearm rotation, this rotation can be: manual, the patient may rotate it; or automatic through a motor.The pronosupinator features, in combination with the arm rehabilitation devices, contribute to the improvement of rehabilitation therapies and reduces costs in general for the healthcare sector.