A manhole security system includes a barrier and an alarm. The barrier fits within a manhole opening and under a manhole cover. The barrier is also formed to substantially close the manhole opening to block access to the manhole if the cover is removed, and includes a basin to collect fluids that may otherwise drain into the manhole. The alarm of the system is positioned on the barrier and includes a monitoring device operable to directly or indirectly monitor the position of the manhole cover and/or to detect a volatile gas in the manhole. Upon the removal of a manhole cover and/or upon the detection of a volatile gas within the manhole, the alarm emits an audible alert and/or transmit a signal to a remote station indicating that the manhole cover has been removed an/or the presence of a volatile gas in the manhole.