Abstract: A valve lock for a tank truck comprising an elongated straight length of strap metal having a shorter leg at each end projecting laterally in the same direction therefrom. The shorter leg at one end is secured to an upstream portion of the valve body which connects to the conduit leading from the tank of the truck, and the shorter leg at the other end is releasably secured to the rotary valve operator or valve handle to prevent its movement from the valve closed position to the valve open position until released. The lock device is positioned in such a way that the elongated portion of strap metal bears firmly against a portion of the valve body, such as its outlet spout, to further restrain rotation of the valve operator or valve handle in the direction of rotation which would open the valve.