A manhole barrier system configured to prevent unauthorized access or dumping or removal of materials into manholes is provided. The manhole barrier system includes a substantially hollow sub-ring configured to be secured to the inside wall of the manhole, preferably using a layer of adhesive, and to receive a pan configured to serve as a barrier therein. The pan may be secured to the sub-ring by way of circumferentially spaced locking assemblies. The locking assemblies are further configured to allow for a limited amount of play between the pan and the sub-ring such that water trapped in the manhole may be allowed to flow upwardly out of the manhole.