NiO/ZnO p–n heterostructures were successfully synthesized by using a hydrothermal method followed by calcination. The morphology of the NiO/ZnO p–n heterostructures could be controlled by the amount of Ni concentration, with 10% Ni the optimum content. The structural features of the NiO/ZnO p–n heterostructures were characterized in detail by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). Additionally, direct current (DC) I–V curves of the NiO/ZnO p–n heterostructures showed diode-like behavior, which is indirect evidence demonstrating that p–n heterojunctions were formed between NiO and ZnO. The 10% NiO/ZnO heterostructures gas sensor exhibited a good gas response, fast response/recovery times and long-term stability to ethanol vapor even at 200 °C, the reduced operating temperature was much lower than for pure ZnO. The decline of the operating temperature was attributed to the formation of p–n heterojunctions. Meanwhile, a possible gas sensing mechanism is illustrated by the calculated energy band positions of the NiO/ZnO p–n heterostructures and alternating current (AC) impedance spectra.