Abstract: A child-resistant safety device for attachment to firearms having a barrel with a muzzle and a cocking mechanism such as a hammer, for temporarily rendering the firearm inoperative, but which may be quickly removed by an adult to enable the firearm to be operated. The device comprises a block adapted to lie against the muzzle of the gun, and a strap having a first portion adapted to extend behind the cocking mechanism of the gun and a second portion adapted to extend from the first portion to the block. Fasteners extend through the block into engagement with the strap to hold it firmly behind the hammer, and a releasable cap is secured on the block through interengaged detents to prevent removal of the cap from the block. A pin carried by the cap extends through the block and into the muzzle of the gun to prevent removal of the cap and block from the muzzle of the gun, and thus prevent removal of the strap from the cocking mechanism.