This paper demonstrates a new kind of hydrogen sensor using palladium (Pd) nanowires. Hydrogen sensors using Pd metal have usually been utilizing the incremental change in electrical resistance of Pd upon hydrogen incorporation. Unlike the conventional Pd hydrogen sensors, however, the electrical resistance of the present Pd nanowire sensor decreases when hydrogen is incorporated into Pd nanowires. It is considered to be due to swelling of the nanowires as the result of hydrogen incorporation and subsequent narrowing of gaps between the nanowires, even though each nanowire should have had the higher resistance inherently. Because of extraordinarily high surface area of nanowires, the performance of sensing the hydrogen concentration was found to superior by far to the conventional Pd sensors. The response and recovery times are quite fast to be about 0.7 and 20 s, respectively and the sensing range of 0.2 similar to 1% hydrogen concentration is suitable for the hydrogen safety sensors. The sensor introduced in this paper is unique with regard to both the sensing mechanism and performance.