A headrest for a motor vehicle seat, said headrest comprising: - a frame for mounting on a backrest of said seat, said frame comprising two parallel vertically extending rods, - a molded plastic main body slidably mounted on said rods so as to be able to adopt a plurality of vertical adjustment positions, - a first device for locking said main body in the selected adjustment position, - a secondary body made of molded plastic material slidably mounted longitudinally on said main body, so that said secondary body can adopt a plurality of longitudinal adjustment positions, - a second device for locking said secondary body in the selected adjustment position when an impact is applied rearwardly to said body, - a common crosspiece for unlocking said devices, said crosspiece being transversely slidably mounted on said main body, said crosspiece having an end provided with a button for unlocking actuation, so as to free sliding of said main and secondary bodies, said crosspiece being provided with an actuating rack, - a first spring means for returning said crosspiece to the locked position, - a pinion is rotatably mounted on said main body along a longitudinal axis, said pinion meshing with said actuating rack, said pinion being provided with a first and a second eccentric, said headrest being characterized in that: - said second locking device comprises a first upper latch and a second lower latch each vertically slidably mounted on said main body, said latches being actuated away from each other by a second spring means interposed between them to dispose them in a locking position where said latches cooperate respectively with a first and a second locking rack extending longitudinally on said secondary body, said latches being operable towards a close unlocking position where they move away from said locking racks to release sliding of said secondary body, - the unlocking actuation of said crosspiece causes said eccentrics to bear against two respective cam surfaces (34a, 34b) provided on each of said latches, so as to bring said latches closer to one another to bring them into the unlocked position. - a frame for mounting on a backrest of said seat, said frame comprising two parallel vertically extending rods, - a molded plastic main body slidably mounted on said rods so as to be able to adopt a plurality of vertical adjustment positions, - a first device for locking said main body in the selected adjustment position, - a secondary body made of molded plastic material slidably mounted longitudinally on said main body, so that said secondary body can adopt a plurality of longitudinal adjustment positions, - a second device for locking said secondary body in the selected adjustment position when an impact is applied rearwardly to said body, - a common crosspiece for unlocking said devices, said crosspiece being transversely slidably mounted on said main body, said crosspiece having an end provided with a button for unlocking actuation, so as to free sliding of said main and secondary bodies, said crosspiece being provided with an actuating rack, - a first spring means for returning said crosspiece to the locked position, - a pinion is rotatably mounted on said main body along a longitudinal axis, said pinion meshing with said actuating rack, said pinion being provided with a first and a second eccentric, - said second locking device comprises a first upper latch and a second lower latch each vertically slidably mounted on said main body, said latches being actuated away from each other by a second spring means interposed between them to dispose them in a locking position where said latches cooperate respectively with a first and a second locking rack extending longitudinally on said secondary body, said latches being operable towards a close unlocking position where they move away from said locking racks to release sliding of said secondary body, - the unlocking actuation of said crosspiece causes said eccentrics to bear against two respective cam surfaces (34a, 34b) provided on each of said latches, so as to bring said latches closer to one another to bring them into the unlocked position.