Rotation mechanism for a forearm which includes a moving stay (2) which supports a wrist, which includes at least one guide track in the form of an arc of a circle, a fixed stay (4) which supports the rear end of a forearm, rods (12) that are rigid lengthwise which link the moving stay (2) and the fixed stay (4) and which allow the moving stay (2) and fixed stay (4) to rotate around a longitudinal axis (Z), by means of articulated links which can rotate (14), and a strut (6) rigidly linked at one of its ends to the fixed stay (4), and linked at its other end to a roller-bearing, whose rollers are in rolling contact with the guide track of the moving stay (2), and which cause the moving stay (2) to pivot around a longitudinal axis (Z), where the direction of said longitudinal axis (Z) passes through the centre of the arc of the circle of the guide track and through the centre of a circle defined by the ends of rods linked to the fixed stay.