An antitheft device for a steering column of a motor vehicle comprising: - a support, comprising: • an antitheft body, and • a collar, fastened to the antitheft body, - a pin, partially housed within the collar, mounted movable in translation in the antitheft device along an axis of translation of the pin, alternately between a locking position according to which said pin blocks the rotation of the steering column and an unlocking position according to which said pin enables the rotation of the steering column, - a contact lock, secured to the support of the antitheft device and arranged so as to displace the pin alternately between said locking position and said unlocking position, said contact lock being adapted to be handled by a user, - a member for deadlocking the pin, mounted in the support of the antitheft device compressed along at least one axis (37, 38) substantially orthogonal to the axis of translation of the pin and held compressed at least by a peripheral wall of said lock, defining a rest position of the antitheft device according to which the deadlocking member enables a displacement of the pin, the deadlocking member being further designed so as to loosen up during a detachment from the contact lock and from the support of the antitheft device and to block the translation of said pin, so as to define a deadlocking position of the antitheft device, the antitheft device being characterized in that the collar of the support of said device includes a groove extending in a direction substantially parallel to the axis of translation of the pin, receiving a portion of the deadlocking member when the antitheft device is in the deadlocking position. - a support, comprising: • an antitheft body, and • a collar, fastened to the antitheft body, • an antitheft body, and • a collar, fastened to the antitheft body, - a pin, partially housed within the collar, mounted movable in translation in the antitheft device along an axis of translation of the pin, alternately between a locking position according to which said pin blocks the rotation of the steering column and an unlocking position according to which said pin enables the rotation of the steering column, - a contact lock, secured to the support of the antitheft device and arranged so as to displace the pin alternately between said locking position and said unlocking position, said contact lock being adapted to be handled by a user, - a member for deadlocking the pin, mounted in the support of the antitheft device compressed along at least one axis (37, 38) substantially orthogonal to the axis of translation of the pin and held compressed at least by a peripheral wall of said lock, defining a rest position of the antitheft device according to which the deadlocking member enables a displacement of the pin, the deadlocking member being further designed so as to loosen up during a detachment from the contact lock and from the support of the antitheft device and to block the translation of said pin, so as to define a deadlocking position of the antitheft device, the antitheft device being characterized in that the collar of the support of said device includes a groove extending in a direction substantially parallel to the axis of translation of the pin, receiving a portion of the deadlocking member when the antitheft device is in the deadlocking position.