An actuator for safety devices (2) for needles of medical instruments of the type comprising two half-parts (3), which are connected together by a hinge line (4) and can be closed about a needle of a medical instrument to render it safe； the half-parts (3) each have an engagement portion (11) set at respective ends (3a) of the half-parts (3) to grip on the needle in a closing configuration of the half-parts (3)； the actuator comprises a support (17) for supporting the half-parts (3) of the safety device (2), and a sliding slot (24) made in the support (17) in which the engagement portions (11) of the half-parts (3) can be slidably inserted； the sliding slot (24) has a width substantially equal to a width of the safety device (2) at the ends (3a) in the closing configuration of the half-parts (3) so as to bring them up to one another and push the half-parts (3) into the closing configuration.
