In this paper, the heterostructure of MoS2/WS2was prepared by a hydrothermal method; the n-n heterostructure was demonstrated using TEM combined with Mott-Schottky analysis. The valence and conduction band positions were further identified by the XPS valence band spectra. The NH3-sensing properties were assessed at room temperature by changing the mass ratio of the MoS2and WS2components. The 50 wt%-MoS2/WS2sample exhibited the best performance, with a peak response of 23643% to NH3at a concentration of 500 ppm, a minimum detection limit of 20 ppm, and a fast recovery time of 2.6 s. Furthermore, the composites-based sensors demonstrated an excellent humidity immune property with less than one order of magnitude in the humidity range of 11–95% RH, revealing the practical application value of these sensors. These results suggest that the MoS2/WS2heterojunction is an intriguing candidate for fabricating NH3sensors.