Abstract: A security lock assembly for a shut-off valve consisting of a valve body and an exterior engageable portion secured to a gate stem so as to rotate the stem. The body has a flange with a hole therein. The security lock assembly comprises a lock disk having a central opening for receiving the engageable portion. Abutment edges are formed integrally with the disk for engagement with the engageable portion so as to rotate therewith. Indentations are formed in a bottom surface of the lock disk at a predetermined position. A pin-lock assembly is retained in the hole of the flange of the valve body. The pin-lock assembly has a pin portion upwardly biased against the lock disk. The pin portion is aligned with the indentation so as to engage therein when the valve stem is rotated to a predetermined closed position, whereby the valve is locked in the predetermined closed position by the pin portion.