A locking collar slideably fits over the control handle of a valve, such as a ball valve, and can be moved to locking and unlocking positions. The collar includes a pair of fork-like tines which a re spaced apart to span a stop member on the valve body. One of the tines includes a tab with a threaded aperture for receiving a threaded adjustment screw. When tightened against the stop member, the screw secures the locking collar and, therefore, the valve handle in a fixed position, preventing inadvertent changing of the valve position. An aperture in the handle allows a lock to be inserted through the handle preventing movement of the locking collar to prevent changing of the valve position by unauthorized tampering.