Abstract: A mechanical device for synchronous movement of a seat and a backrest of a chair comprising a first supporting bracket which is associated with a central column which protrudes from a rotating base and is pivoted transversely to an end of a second fixing bracket for the seat. The second bracket cooperates with two pairs of slotted laminar elements which are pivoted to the first bracket. A third anchoring bracket for the backrest is also provided which is pivoted to the first bracket and to a pair of linkages which are in turn pivoted to the second bracket. Moreover, a piston is present between the third and first brackets and a pair of springs is present between the first and second brackets.