Hierarchically-structured nanospheres of spinel MnFe2O4 were synthesized by a simple solvothermal method. With a diameter in the range of 200-300 nm, each nanosphere is a spherical aggregate of tiny crystallites sized on the order of a few nanometers. The surface layers of the nanospheres are porous with roughness on the order of dozens of nanometers. Semiconductor gas sensors based on the MnFe2O4 nanospheres demonstrate excellent NO2 sensing performance at 200 degrees C, including high sensitivity with a very low detection limit of 25 ppb, rapid response and recovery, as well as high selectivity. Such outstanding performance can be attributed to the electron-capturing chemisorption of NO2 facilitated on the nano-porous surfaces of the MnFe2O4 nanospheres.