Room-temperature (RT) gas sensors for nitrogen dioxide (NO₂) detection face persistent challenges, including reliance on high operating temperatures and inefficient charge carrier utilization under UV activation. To address these limitations, we engineered a p-n nano-heterojunction (NHJ) gas sensor by integrating p-type nickel oxide (NiO) nanoparticles onto n-type zinc oxide (ZnO) nanorods. This architecture leverages UV-driven carrier generation and interfacial electric fields at the NHJ to suppress recombination, enabling unprecedented RT performance. By optimizing thermal annealing conditions, we achieved a well-defined heterojunction with uniform NiO distribution on the top of the ZnO nanorods, validated through electron microscopy and X-ray photoelectron spectroscopy. The resulting sensor exhibits a 5.4-fold higher normalized response to 50 ppm NO₂ under 365 nm UV illumination compared to pristine ZnO, alongside rapid recovery and stable cyclability. The synergistic combination of UV-assisted carrier generation and heterojunction-driven interfacial modulation offers a promising direction for next-generation RT gas sensors aimed at environmental monitoring.