Hydrogen-related technologies are rapidly developing, driven by the necessity of efficient and high-density energy storage. This poses new challenges to the detection of dangerous gases, in particular the realization of cheap, sensitive, and fast hydrogen sensors. Several materials are being studied for this application, but most present critical bottlenecks, such as high operational temperature, low sensitivity, slow response time, and/or complex fabrication procedures. Here, we demonstrate that WO3 in the form of single-crystal, ultrathin films with a Pt catalyst allows high-performance sensing of H2 gas at room temperature. Thanks to the high electrical resistance in the pristine state, this material is able to detect hydrogen concentrations down to 1 ppm near room temperature. Moreover, the high surfaceto-volume ratio of WO3 ultrathin films determines fast sensor response and recovery, with characteristic times as low as 1 s when the concentration exceeds 100 ppm. By modeling the hydrogen (de)intercalation dynamics with a kinetic model, we extract the energy barriers of the relevant processes and relate the doping mechanism to the formation of oxygen vacancies. Our results reveal the potential of single-crystal WO3 ultrathin films toward the development of sub-ppm hydrogen detectors working at room temperature.