A mechanism (10, 110, 210, 310) for tilting the seat (3) and / or backrest (2) of a chair, the mechanism (10, 110, 210, 310) comprising: - a frame (11, 111, 211, 311), adapted to be associated with a stem (12) of a gas-filled cylinder (13), - a backrest support (14, 114, 214, 314), coupled to said frame (11, 111, 211, 311), - a seat support (15, 115, 215, 315), coupled to said frame (11, 111, 211, 311) and to said backrest support (14, 114, 214, 314), - elastic return means (16, 116, 216, 316) for the elastic return of said backrest support (14, 114, 214, 314). The elastic return means (16, 116, 216, 316) are arranged between the frame (11, 111, 211, 311) and the backrest support (14, 114, 214, 314).