Abstract: A mechanism for adjusting the seat of a chair as a result of changing the tilt of the backrest with respect to the seat is provided. The mechanism has a backrest frame and a base frame, as well as a covering element stably combined with the base frame and forming part of the seat. The backrest frame is rotatably constrained to the base frame around a respective rotation axis and tilting with respect to the seat between a position (P1) of minimum tilt and a position (P2) of maximum tilt. Advantageously, the covering element is partially elastically deformable, with the mechanism having deforming means operated by the backrest frame in order to reversibly deform, in the vertical direction, when the covering element as a function of the tilt angle with backrest frame has reached with respect to the position (P1) of minimum tilt.