Anti-theft device for coupling

An anti-theft device (1) for a coupling comprising a coupling head (TA) having a cavity (CA) for securing a trailer, said anti-theft device (1) comprising a first element (11), called lower, disposed beneath said coupling head, and a second element (12), called upper, disposed above said coupling head, said elements (11, 12) being secured together by means of clamping means (16, 17) so as to clamp said head and form a housing (10) around said head, locking means comprising a flange (14) capable of presenting two positions (P1, P2) of which a first position, called unlocked, in which the cavity is capable of receiving the coupling ball (BA), and a second position, called locked, in which the flange is protruding into the cavity.