An anti-theft device for vehicles steered by means of a handlebar fixed to a fork (2, 3) accepting a steered wheel, comprising first means (12) of connection with the handlebar and second means (4, 5, 6) of connection with the fork, coupling means (7, 11, 18, 19, 20, 24) between the said first and second connection means, a two-position actuation device (21, 22, 23) locking any relative movement between the first and second connection means in the position of use of the vehicle and allowing free rotation between the said first and second connection means in the engaged position of the said anti-theft device, the actuation device comprising an interlocking mechanism (18, 21, 26), locked when the actuation device is in the anti-theft position and activated when the actuation device is in the position of use, independently of the relative positions of the handlebar and fork.