The present invention relates to a seat for vehicles, comprising a backrest part with a support frame and a respective lining; a seat part; and a chassis to which the support frame of the backrest part is firmly attached and to which the seat part is attached such that it can shift forward or backward, the backrest part being provided with a rotating vane between the support frame and its lining, with an upper end attached in an articulated manner to the frame or to the chassis and with a lower end provided with a slide where a thrust pin arranged in the rear area of the seat part slides, such that the rotation of the vane with respect to the frame or the chassis shifts the seat and modifies the profile of the lining in the area covering the vane and therefore the shape of the support surface that the backrest part offers the user.