A composite consisting of polyaniline (PANI) and reduced graphene oxide (rGO) was synthesized by in situ polymerization of the monomer aniline in the presence of rGO under acidic conditions. The PANI-rGO composite substrate was doped with Pd nanoparticles via chemical reduction. The resulting Pd PANI-rGO nanocomposite was characterized by transmission electron microscopy, scanning electron microscopy, X-ray diffraction, and Fourier-transform infrared spectroscopy before it was utilized to fabricate a hydrogen sensor. Compared with the sensor based on PANI-rGO or PANI, the Pd PANI-rGO sensor was highly sensitive and selective to hydrogen gas, with fast response time in air at room temperature. The significantly enhanced sensitivity resulted from the faster spill-over effect, dissociation of hydrogen molecules on Pd, and the high surface area of the PANI-GO composite. Based on its improved sensing properties, ease of fabrication, and stable operation, the Pd PANI-rGO nanocomposite shows promise for high-performance hydrogen-sensing applications. Copyright (C) 2016, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.