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SnO₂-core/ZnO-shell nanowires were synthesized using a two-step process: the synthesis of SnO₂ nanowires by the thermal evaporation of Sn powders followed by the atomic layer deposition of ZnO. The room temperature NO₂ gas sensing properties of the nanowires under ultraviolet (UV) illumination were examined. The cores and shells of the nanowires were primitive tetragonal-structured single crystal SnO₂ and wurtzite-structured single crystal ZnO, respectively. The responses of multiple networked SnO₂ nanowire sensors were increased 2–3-fold at NO₂ concentrations ranging from 1 to 5 ppm by encapsulating the nanowires with ZnO. The SnO₂-core/ZnO-shell nanowire sensors showed a remarkably enhanced response under UV illumination. The sensing mechanism of the core/shell nanowires under UV illumination is also discussed.