This paper reported a p-NiO/n-ZnO heterostructures synthesized by a low-cost and environment-friendly strategy. X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive X-ray (EDX) spectroscopy were used to characterize the as-synthesized powders. The results indicated that the p–n heterostructures formed between the cubic structure NiO and hexangular structure ZnO in the NiO/ZnO composites. Furthermore, the gas sensing performance for detection of acetone and photocatalytic activity for the degradation of methyl orange (MO) of the NiO/ZnO composites was investigated. The composites showed much higher performance than that of pure NiO and ZnO, which could be ascribed to the formation of p–n heterojunctions in the NiO/ZnO composites.