Heterojunctions are very promising structures due to their hybrid properties, which are usually obtained via a multistep growth process. However, in this paper, WO3-W18O49 heterostructures are synthesized via a novel one-step approach by using isopropanol as reaction media and are applied in NH3 gas detection for the first time. The obtained WO3-W18O49 heterostructures with loose nanowire bundle-like morphology show a response value of 23.3 toward 500 ppm NH3 at 250 degrees C, which is 5.63 times higher than that of pristine W18O49. In addition, the WO3-W18O49 sensor also exhibits great dynamic response/recovery characteristics (13 s/49 s @ 500 ppm NH3), superb selectivity and low detection limit of 460 ppb. The substantial improvement in the response of WO3-W18O49 heterostructures toward NH3 can be explained by the formation of n-WO3/n-W18O49 heterojunctions that facilitate the generation of a more extended depletion layer as well as the enhancement of specific surface area and pore volume. Our research results open an easy pathway for facile one-step preparation of heterojunctions with high response and low cost, which can be used for the development of other high-performance gas sensors.