A pressure relief valve monitoring system and method is disclosed. In one aspect there is an online pressure relief valve monitoring system that includes at least one acoustic sensor located about a pressure relief valve that obtains ultrasonic measurements therefrom. One or more temperature sensors located about the pressure relief valve obtain temperature measurements therefrom. A data acquisition system receives the ultrasonic measurements from the at least one acoustic sensor and the temperature measurements from the one or more temperature sensors. An online monitor monitors the measurements received by the data acquisition system for detection of a leak about the pressure relief valve. The online monitor detects the presence of the leak as a function of the ultrasonic measurements and temperature measurements.