The present disclosure relates to a safety mechanism (5) for a push-off cap of a medicament delivery device. The safety mechanism (5) includes a cap (9) and an actuator assembly (11). The cap (9) has a through-opening (9b) and is configured to be mounted to the proximal end of a medicament delivery device. The actuator assembly (11) is configured to be received by the cap (9), and extend proximally from the cap (9). The actuator assembly (11) is linearly displaceable relative to the cap (9), from a first position in which the actuator assembly (11), to a second position in which the actuator assembly (11) is displaced in the distal direction and received further by the cap relative to the first position. The actuator assembly (11) includes a guide structure configured to guide linear displacement of the actuator assembly (11) relative to the cap (9), a proximal top member (11c) and a lock structure (11d). The proximal top member (11c) is pivotally arranged relative to the guide structure (11a), between a first pivot position and a second pivot position. The lock structure (11d) extends distally, from a distal surface of the proximal top member (11c), from the proximal top member to the proximal end surface of the cap. By pivoting the proximal top member (11c) relative to the guide structure (11a), the lock structure (11d) which is pivotally fixed relative to the proximal top member, will also be pivoted. The proximal top member (11c) is pivotable from the first pivot position in which the lock structure (11d) is configured to be bear against the proximal end surface (9a) of the cap (9) to the second pivot position, in which the lock structure (11d) aligns with the through-opening (9b) of the cap (9).