A fiber optic-based irreversible moisture sensor that can be used to determine if a location has ever been wet during a given monitoring period is presented. The irreversible sensor offers a simple means to detect moisture when more complex dedicated continuous monitoring is not feasible and/or where traditional moisture sensors are not applicable, such as in confided spaces. The irreversible moisture sensor has three response states that indicate if an area is actively wet, has been wet but is currently dry, or has never been wet. The irreversible moisture sensor is based on long period grating fiber optic technology. The functionality and survivability of the sensor to exposure in typical atmospheric environments has been evaluated. Testing consisted of long-term exposure to low and high humidity levels (<10 to 97 % relative humidity), and exposure to freezing and elevated temperatures (-10 and 60 °C). The sensors were monitored periodically to examine their functionality. The sensors exhibited rapid irreversible response to water even after extended exposure to the simulated service conditions, and thus are suitable for long-term service in typical atmospheric environments. Potential applications for this sensor include remote, confined or difficult to access locations, aggressive environments where electrical-based sensors perform inadequately, or where approval for fielding instrumentation for continuous monitoring maybe prohibitive, such as on aircraft.