The invention relates to a vibrating warning device for a vehicle seat, having a vibrating patch (31) intended to be attached to the upper face of the padding of the seat part or backrest of one said seat, said patch (31) comprising at least one electrically controlled vibratory actuator (32) accommodated in a flexible support jacket (33); characterized in that said flexible support jacket (33) has a main body (34) enclosing said vibratory actuator(s) (32), and at least two arms (39) extending from said main body (34), on either side of the latter, and each comprising a retaining hook (40) intended to engage with a corresponding recess made in said padding in order to keep said main body (34) against the upper face of this padding.