A formaldehyde gas sensor with high sensitivity and superior selectivity has been fabricated successfully with NiO–SnO₂ hybrid nanospheres, which consisted of n-type porous SnO₂ nanospheres and p-type NiO dopants. It has been characterized by X-ray diffraction (XRD), high-resolution electron microscopy (HRTEM), Scanning Electron Microscopy (SEM), X-ray photoelectron spectroscopy (XPS), Brunauer–Emmett–Teller (BET) and energy-dispersive X-ray spectroscopy (EDS). Compared with pure SnO₂, the NiO-doped SnO₂ nanospheres sensor exhibited significantly enhanced formaldehyde-sensing performances, including lower optimum sensing temperature (about 100 °C), lower detectable threshold (0.5 ppm), higher sensitivity (response = 26.03 to 50 ppm HCHO) and superior selectivity. Based on our experimental results, the role of the addition of NiO and a possible sensing mechanism has also been discussed.