Abstract: A security locking device for dead-bolt door locks which is distinguished by a shroud that encompasses and grips the dead-bolt actuator at the inside of the door, and is rotatably mounted in a base secured to the inner face of the door, with a handle portion on the shroud exposed to enable the shroud -- and hence the dead-bolt actuator -- to be manually turned. A spring biased security bolt slidably mounted in the base engages a cylindrical side surface of the shroud and snaps into a keeper recess in that surface when the shroud is turned to the position at which the dead-bolt is in its projected door-locking position. A manually rotatable cylinder with an eccentric lug at its inner end to engage one or the other of a pair of opposing shoulders on the security bolt is mounted in the base and operable from inside the door to enable retraction of the security bolt and to selectively hold the security bolt in either its projected operative position or its retracted inoperative position.