Abstract: A system for locking a chain-operated valve to prevent unauthorized and inadvertent operation of the valve. The system is specifically advantageous for use with valves which cannot be easily accessed, i.e. in an overhead space, and therefore are operated by an elongated chain encircling a valve handle to permit remote operation by pulling the chain to rotate the valve in the appropriate direction for opening or closing. The system includes an elongated tube for receiving both lengths of a chain encircling the valve and a locking arrangement positioned at one end of the tube. Both lengths of the chain extending from the valve enter one end of the tube and exit the opposite end of the tube adjacent the lock arrangement. The lock arrangement includes a bracket secured to the tube and extending outwardly from the tube. The bracket preferably includes apertures for receiving a lockout hasp extending through the chain links and the bracket.