Here, we report a Pd/PdO_x sensing material that achieves 1-s detection of 4% H₂ gas (i.e., the lower explosive limit concentration for H₂) at room temperature in air. The Pd/PdO_x material is a network of interconnected nanoscopic domains of Pd, PdO, and PdO₂. Upon exposure to 4% H₂, PdO and PdO₂ in the Pd/PdO_x are immediately reduced to metallic Pd, generating over a > 90% drop in electrical resistance. The mechanistic study reveals that the Pd/PdO₂ interface in Pd/PdO_x is responsible for the ultrafast PdO_x reduction. Metallic Pd at the Pd/PdO₂ interface enables fast H₂ dissociation to adsorbed H atoms, significantly lowering the PdO₂ reduction barrier. In addition, control experiments suggest that the interconnectivity of Pd, PdO, and PdO₂ in our Pd/PdO_x sensing material further facilitates the reduction of PdO, which would otherwise not occur. The 1-s response time of Pd/PdO_x under ambient conditions makes it an excellent alarm for the timely detection of hydrogen gas leaks.