Beside the noble metal nanoparticles (NPs), plasmonic semiconductors are being developed nowadays for a variety of applications such as catalyst, phototherapy and sensing technologies. In the present study, colloidal plasmonic MoO3 NPs with <20 nm mean size are prepared by flame synthesis as a rapid, cost effective, catalyst-free and atmospheric route. A Mo rod was converted to oxide nanopowder in an oxyhydrogen flame and collected on a water-cooled rotating substrate. Colloidal solutions of these particles were made by dispersing the MoO3 NPs into DI water. As a palladium precursor, solutions of aqueous PdCl2 were added to the colloidal NPs to make them sensitive to hydrogen. Formation of cubic Pd NPs in among the MoO3 NPs were recognized in TEM images for hydrogen-exposed samples. Upon hydrogen injection, a localized surface plasmon resonance (LSPR) absorption band was appeared in the UV-Vis absorption spectra at 700-800 nm region, which began to decline by further exposure time. Within few seconds of gas exposure, samples turned blue and longer exposure made them brownish. The LSPR peaks exhibited a spectral blue shift with a more hydrogenation, in agreement with the existing plasmonic model. In addition, a correlation between optical band gap and LSPR intensity was found. The colloidal Pd-MoO3 solutions then were employed for LSPR sensing of hydrogen at a concentration range of 0-5%. A pronounced LSPR linear sensitivity toward low concentration (<2.5%) of hydrogen was observed. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.