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 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 longitudinally slidably mounted 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, said device comprising a latch mounted for vertical sliding on said main body between a locking position, in which it cooperates with a rack extending longitudinally on said secondary body, and an unlocking position, in which it moves away from said rack to release sliding of said secondary body, • a crosspiece for simultaneous unlocking of said devices, said crosspiece being transversely slidably mounted on said main body, said crosspiece having an end provided with a knob enabling it to be actuated in translation towards an unlocking position, so as to free sliding of said main and secondary bodies, • spring means for returning the crosspiece to the locked position, said headrest being characterized in that it further comprises a force return cam arranged between said crosspiece and said latch, said cam presenting in longitudinal view: • a point of rotational connection to said crosspiece along a longitudinal axis of rotation, • and, laterally on either side of said point: o on the one hand, a first finger slidably mounted in a guide slot provided in said main body and a locking edge of said latch in locking position, o and secondly an actuating edge forming a cam surface bearing on said latch, said slot being arranged in such a way as to induce, during the displacement of said crosspiece in unlocking and the resulting transverse displacement of said point: • a displacement of said locking edge releasing actuation of said latch in unlocking mode, • and rotation of said cam about said axis, said rotation effecting a vertical displacement of said actuating edge pushing said latch towards its unlocked position. • a frame for mounting on a backrest of said seat, said frame comprising two parallel vertically extending rods, • a 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 longitudinally slidably mounted 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, said device comprising a latch mounted for vertical sliding on said main body between a locking position, in which it cooperates with a rack extending longitudinally on said secondary body, and an unlocking position, in which it moves away from said rack to release sliding of said secondary body, • a crosspiece for simultaneous unlocking of said devices, said crosspiece being transversely slidably mounted on said main body, said crosspiece having an end provided with a knob enabling it to be actuated in translation towards an unlocking position, so as to free sliding of said main and secondary bodies, • spring means for returning the crosspiece to the locked position, said headrest being characterized in that it further comprises a force return cam arranged between said crosspiece and said latch, said cam presenting in longitudinal view: • a point of rotational connection to said crosspiece along a longitudinal axis of rotation, • and, laterally on either side of said point: o on the one hand, a first finger slidably mounted in a guide slot provided in said main body and a locking edge of said latch in locking position, o and secondly an actuating edge forming a cam surface bearing on said latch, • a point of rotational connection to said crosspiece along a longitudinal axis of rotation, • and, laterally on either side of said point: o on the one hand, a first finger slidably mounted in a guide slot provided in said main body and a locking edge of said latch in locking position, o and secondly an actuating edge forming a cam surface bearing on said latch, o on the one hand, a first finger slidably mounted in a guide slot provided in said main body and a locking edge of said latch in locking position, o and secondly an actuating edge forming a cam surface bearing on said latch, said slot being arranged in such a way as to induce, during the displacement of said crosspiece in unlocking and the resulting transverse displacement of said point: • a displacement of said locking edge releasing actuation of said latch in unlocking mode, • and rotation of said cam about said axis, said rotation effecting a vertical displacement of said actuating edge pushing said latch towards its unlocked position. • a displacement of said locking edge releasing actuation of said latch in unlocking mode, • and rotation of said cam about said axis, said rotation effecting a vertical displacement of said actuating edge pushing said latch towards its unlocked position.