A three-dimensional (3D) hierarchical structure consisting of vertically aligned, double-sided, and self-supported WO₃ nanocolumn bundles has been successfully synthesized via an acid-assisted (HCl) hydrothermal process without any templates, catalysts, or substrates. A possible formation mechanism is proposed, which involves dissolution–recrystallization and Ostwald ripening processes in concert with the structure-directing role of HCl, leading to the morphology grown along [200] with largely exposed (002) facets. The hierarchical monoclinic structure of the WO₃-based sensor contains abundant active sites and loose structures which afford beneficial conditions for gas adsorption and diffusion, and the as-synthesized sensor proves to be an excellent NO₂-sensing material with high sensitivity, good selectivity, rapid response (ca. 23 s)/recovery (ca. 11 s), and remarkable repeatability at a low operating temperature (∼110 °C). A possible gas-sensing mechanism will be discussed based on largely exposed (002) facets, the O-terminated (001) surface of which containing unsaturated coordinated O atoms is more active to adsorb NO₂ molecules easily and efficiently. The superior gas sensor properties offer a potential platform for monitoring harmful and toxic gases, especially those flammable and explosive volatile organic compounds (VOCs).