We have fabricated sensors with reduced graphene oxide (RGO) nanosheets (NSs)-loaded ZnO nanofibers (NFs) via an electrospinning method. The RGO NSs-loaded ZnO NFs were comprised of nanograins with an average diameter of 20 nm. Transmission electron microscopy and X-ray diffraction both revealed the presence of RGO NSs in the ZnO NFs. The sensing properties of RGO NSs-loaded ZnO NFs were examined after exposure to various gases, including O2, SO2, NO2, CO, C6H6, and C2H5OH. The sensor responses showed a bell-shaped behavior with respect to the weight ratio of RGO NSs. It is remarkable that our sensors exhibited significantly higher responses than pure ZnO NFs. We propose a novel hybrid sensing mechanism for the drastic improvement in the sensing behavior that is caused by loading RGO NSs into ZnO NFs. This hybrid sensing mechanism combines the resistance modulation of ZnO/ZnO homointerfaces and RGO-NSs/ZnO heterointerfaces in addition to the radial modulation of the surface depletion layer of ZnO NFs. In the heterointerfaces, the creation of local heterojunctions plays a significant role in raising the sensitivity of RGO-loaded ZnO NFs.