A generally wedge-shaped vibration damper for rotor blades of a gas turbine engine has a pair of scrubbing surfaces and is loosely retained in a pocket incorporated in the rotor blade platform. The pocket is partly defined by a plurality of surfaces which orient the damper so that it is slidably displaceable and rotatable only in a predetermined plane. During rotation of the rotor disk, the damper of one rotor blade is displaced in a plane transverse to the rotor axis of rotation by centrifugal forces to a position at which one scrubbing surface of the damper abuts a side surface of the platform of the adjacent rotor blade and the other scrubbing surface abuts an inner surface of the platform of the rotor blade in which the damper is inserted. The scrubbing action of the damper serves to damp vibratory motion in the platforms of both blades.