In this study, an ultrasonic optical fiber hydrogen sensor was newly developed for monitoring and detecting hydrogen leakages. Previously, we developed a pulse-echo sensor system for health monitoring system. Ultrasonic wave, generated from a piezoelectric actuator, is guided and propagated through the optical fiber and subsequently sensed by a piezoelectric sensor in the pulse-echo sensor system. For the detection of hydrogen, the optical fiber was coated with palladium particles, which expanded on exposure to hydrogen. Palladium was used, because it adsorbs hydrogen gas and swell slightly to form palladium hydride with exposure to hydrogen. The change in palladium dimensions affects the wave propagation impedance of optical fiber for detecting the hydrogen gas, and this method has been applied to the pulse-echo sensor systems. Our results showed that the ultrasonic-optical fiber hydrogen sensor can be successfully employed for hydrogen detecting system.