A roll bar (1′) for a motor vehicle is proposed according to the present invention, with an unsplit torsion bar (5′), which is rotatably fastened to a vehicle chassis (3), is connected to two opposite wheel suspensions (4′) of the said axle of the vehicle, and has a torsion area for the elastic coupling of spring action movements of the opposite wheel suspensions (4′), wherein the roll bar (1′) has, furthermore, a first torsion tube (8′), which surrounds the unsplit torsion bar (5′) at least in a first partial area (B′) of its torsion area, and whose first end (8a′) is connected to the unsplit torsion bar (5′), rotating in unison therewith, and whose second end (8b′) can be connected to the unsplit torsion bar (5′) by means of a clutch arrangement (9′) in such a way that it selectively rotates in unison therewith, so that the torsional stiffness of the roll bar (1′) can be varied.