Adjustment device for a chair with oscillating mechanism

An adjustment device for a chair, comprising an oscillating mechanism composed of a first plate, connected to a central gas-operated post for height adjustment of a seat, to which a second plate, fixed to a backrest, is articulated so that the second plate oscillates by means of a first pivot, wherein a third plate is articulated and interposed, respectively, by means of a second pivot and a third pivot, between said first plate and said second plate, and is configured to be connected to said seat, said first plate having means for connection to at least one slat or leaf spring which is adapted to act against an inclination of said second plate with respect to said first plate, wherein said first plate, which is substantially L-shaped, has a first wing, which is flat and to which said central post is connected, and a second wing, which is elastically deformable and interacts with means for adjusting inclination thereof with respect to said first wing.