H2S gas sensors were fabricated using p-n heterojunctions of NiO/ZnO, in which the ZnO nanorod arrays were wrapped with nickel oxide (NiO) nanosheets via a hydrothermal synthesis method. When the H2S gas molecules were adsorbed and then oxidized on the surfaces ZnO, the free electrons will be released. The increase of electron concentration on the ZnO boosts the transport speeds of electrons on both sides of the NiO/ZnO p-n junction, which significantly improved the sensing performance and selectivity for H2S detection, if compared to those using the pure ZnO nanorod arrays. The response toward 20 ppm of H2S was 21.3 at 160 oC for the heterostructured NiO/ZnO sensor, while the limit of detection was 0.1 ppm. Additionally, it was found that when the sensor was exposed to H2S at an operating temperature below 160 oC, the resistance of sensor was significantly decreased, indicating its n-type semiconductor nature, whereas when the operating temperature was above 160 oC, the resistance was significantly increased, indicating its p-type semiconductor nature. Finally, the sensing mechanism of the NiO/ZnO heterostructured H2S gas sensor was discussed in detail.