Abstract: A device for balancing the forces acting on a robot arm (10) hinged to rotate about an axis (A), said device comprising an even number of auxiliary parts (24.sub.1, 24.sub.2) hinged about respective mutually parallel axes (B.sub.1, B.sub.2) which are also parallel to the axis (A) of the arm, with each of said parts supporting two transverse arms (28,30) whose ends are suitable for bearing against a rectilinear portion (48) of a moving member (50) subjected to a return force towards an initial position, the auxiliary parts (24.sub.1, 24.sub.2) being driven at angular speeds which are equal in absolute value but which are in opposite directions in pairs, said speeds also being equal to one half of the angular speed of the robot arm.