An inexpensive, simple, effective, and easily implementable locking device for use with directional control valves that allows for the valve to be securely locked in a desired operating position, thereby preventing unauthorized, inadvertent or otherwise undesired manipulation of the valve minimizing potentially damaging or injurious conditions. The locking device has an adjustable locking plate and a fixed locking bracket, both of which are adapted to the valve using the valve's existing components for ease of installation and adaptation and so the normal operation of the valve is not impeded. The adjustable locking plate is mounted to the valve over the valve stem. The fixed locking bracket mounts to the valve body using the valve's existing bolts and nuts. Locking holes in the adjustable locking plate and fixed locking bracket line up and allow for the valve to be securely locked in a desired operating position with a locking member.