<p><em>Air pollution caused by toxic gases has become a major concern for human health and the environment. Among the many hazardous gases, Nitrogen dioxide (NO2) has been identified as the most detrimental, causing respiratory, lung, and cardiovascular diseases even at low concentrations. Therefore, the development of low-cost and environmentally friendly sensors that can operate at moderate temperatures and consume minimal electric power is highly desirable. In this study, we developed a highly sensitive and selective NO2 gas sensor by depositing zinc oxide (ZnO) nanoballs on porous silicon (PS) substrates using the radio frequency magnetron sputtering technique. The PS substrates were synthesized at room temperature via metal assisted chemical etching technique to enhance the surface-to-volume ratio and provide high active surface area. The ZnO nanoballs sensor chip exhibited a high sensing response of 29.2% with fast response and recovery times of 70 sec and 96 sec, respectively, to 20 ppm NO2 in dry air at 250°C. The sensor also demonstrated high stability and reproducibility for up to four months and six cycles, respectively, and was capable of detecting NO2 down to 2 ppm at 250°C. The study provides a novel approach for designing a highly repeatable sensor with notable performance for low (ppm) detection of NO2 in the environment at lower operating temperature regimes.</em></p>