A device for adjusting the tilt of a chair backrest having a backrest frame rotatable with respect to the base frame of the chair is provided. The device includes elastic means for returning the backrest and/or countering the rotation thereof, in addition to means for controlling the preload of the elastic means. The means for controlling the preload include at least one first connecting rod and at least one first crank. The first connecting rod is hinged to the first crank and constrained to the elastic means so as to move the point of application of the returning and/or countering force with respect to the backrest. Adjusting means to adjust the position of the first crank determine at least two distinct stable positions of the first crank corresponding to two distinct positions of the mentioned point of application of the returning and/or countering force with respect to the base frame.