A seat backrest for a motor vehicle has a structure, which supports a padding covered by a cover; the structure has a rear plate provided with a coupling portion, to which an end of the cover is fixed along a peripheral edge of the plate itself; the plate is fixed to a front frame by means of a snap-on connecting device having a plurality of deformable appendages, which are carried by the plate in positions that are spaced apart from one another; the appendages are part of respective brackets, which are fixed to the plate and have respective stiffening portions, which are arranged at the coupling portion.