Headrest for a motor vehicle seat, said headrest comprising: • a metal frame for connection to a backrest of said seat, • a housing of moulded plastic material for absorbing energy in the event of an impact of a passenger's head on said headrest, said housing being rotatably mounted in relation to said frame about a transverse axis so as to be able to move from a raised position of use to a folded position of retraction, • a padded cushion of said housing, • a device for locking said housing in a raised position, • a first spring means for actuating said housing towards its folded position when said device is disabled, said headrest being characterised by: • a locking crosspiece extending transversely and being transversely slidably mounted in said housing such as to be able to move from a locking position to an unlocking position, said crosspiece being integral with a locking means, • a reciprocal locking means integral with said frame and cooperating with said locking means to maintain said housing in a raised position, • a second spring means for actuating said crosspiece towards its locking position, • a solenoid for actuating said crosspiece to an unlocking position, said solenoid being mounted in said housing, and in that: • said crosspiece is made of metal, which can be attracted by a magnet, • said crosspiece is mounted so as to pass through the inside of said solenoid and to be able to slide therein, • the power supply terminals of said solenoid are defined such that said crosspiece assumes: -- its locking position in the absence of power under the effect of said second spring means, -- its unlocking position in the presence of power, against said second spring means. • a metal frame for connection to a backrest of said seat, • a housing of moulded plastic material for absorbing energy in the event of an impact of a passenger's head on said headrest, said housing being rotatably mounted in relation to said frame about a transverse axis so as to be able to move from a raised position of use to a folded position of retraction, • a padded cushion of said housing, • a device for locking said housing in a raised position, • a first spring means for actuating said housing towards its folded position when said device is disabled, said headrest being characterised by: • a locking crosspiece extending transversely and being transversely slidably mounted in said housing such as to be able to move from a locking position to an unlocking position, said crosspiece being integral with a locking means, • a reciprocal locking means integral with said frame and cooperating with said locking means to maintain said housing in a raised position, • a second spring means for actuating said crosspiece towards its locking position, • a solenoid for actuating said crosspiece to an unlocking position, said solenoid being mounted in said housing, and in that: • said crosspiece is made of metal, which can be attracted by a magnet, • said crosspiece is mounted so as to pass through the inside of said solenoid and to be able to slide therein, • the power supply terminals of said solenoid are defined such that said crosspiece assumes: -- its locking position in the absence of power under the effect of said second spring means, -- its unlocking position in the presence of power, against said second spring means. -- its locking position in the absence of power under the effect of said second spring means, -- its unlocking position in the presence of power, against said second spring means.