The present invention relates to a secure barrier for an underground valve box. The device may have a bolt and a nut for adjusting the length of the bolt to position curved plates to press against the inner circumference of a cylindrical valve box. The bolt housing may include a flange that protrudes through a slot in a horizontal metal plate. The plate may have handles for ease of lifting and replacement of the plate. The protruding flange may feature a hole for insertion of a padlock. When the padlock is secured, the plate cannot be removed from the cylindrical valve box as the adjustable bolt housing cannot move sufficiently to retract the curved plates from the inner cylindrical surface of the valve box. Thus, only authorized personnel with the proper padlock key or combination can operate the valve, deterring vandals.