An assembly of an extractable sliding seat and an adjustable-conformation backrest for upholstered seats such as sofas or armchairs wherein it comprises: - a seat comprising a lower fixed frame provided with means for resting on the ground and an upper movable frame slidable with respect to said fixed frame, said upper movable frame being capable of translating with respect to said lower fixed frame by passing from a fully retracted position, in which said movable frame does not protrude with respect to the outline of the underlying fixed frame, to a fully extracted position, in which said movable frame protrudes with respect to the front edge of said fixed frame; - a backrest comprising a lower half-frame and at least two upper movable elements, each of which is rotationally connected, according to a respective axis substantially parallel to the transverse axis of the seat, independently of the other, to the upper portion of the lower half-frame, the latter being connected to the rear portion of the movable frame of said seat by means of a kinematic mechanism which causes the lower half-frame to rotate backward as the movable frame of the seat translates forward with respect to the fixed frame; - a kinematic mechanism integrally connected to the lower fixed frame of said seat and associated with said movable frame and the lower half-frame of said backrest by means of connecting means which allow the forward translation of said movable frame with respect to said fixed frame simultaneously causing the backward rotation of said lower half-frame with respect to said movable frame, according to an axis substantially parallel to the transverse axis of the seat. - a seat comprising a lower fixed frame provided with means for resting on the ground and an upper movable frame slidable with respect to said fixed frame, said upper movable frame being capable of translating with respect to said lower fixed frame by passing from a fully retracted position, in which said movable frame does not protrude with respect to the outline of the underlying fixed frame, to a fully extracted position, in which said movable frame protrudes with respect to the front edge of said fixed frame; - a backrest comprising a lower half-frame and at least two upper movable elements, each of which is rotationally connected, according to a respective axis substantially parallel to the transverse axis of the seat, independently of the other, to the upper portion of the lower half-frame, the latter being connected to the rear portion of the movable frame of said seat by means of a kinematic mechanism which causes the lower half-frame to rotate backward as the movable frame of the seat translates forward with respect to the fixed frame; - a kinematic mechanism integrally connected to the lower fixed frame of said seat and associated with said movable frame and the lower half-frame of said backrest by means of connecting means which allow the forward translation of said movable frame with respect to said fixed frame simultaneously causing the backward rotation of said lower half-frame with respect to said movable frame, according to an axis substantially parallel to the transverse axis of the seat.