In this work, a YSZ mixed-potential sensor was fabricated utilising square-surface nanostructured tungsten oxide (WO3) as an NO2 gas sensing-electrode. YSZ (ZrO2+8% mol Y2O3) was used as the high temperature electrolyte layer of the sensor. The square-surface nanostructured WO3 was synthesised by a hydrothermal treatment and used to fabricate the NO2 gas sensing-electrode. The sensor with a configuration of Pt/YSZ/(Pt-WO3) was investigated at operating temperatures ranging from 320 to 600oC to monitor sensing performance to NO2, NO, CO, CH4, and CO2 gases. The Pt/YSZ/(Pt-WO3) sensor presented a high selectivity to NO2 gas. This behaviour is suggested to occur due to the specific high catalytic activity of the square-surface nanostructured WO3 to NO2 gas. The highest gas-sensing response (∆Uemf) of the sensor was about 40.9 mV for 50 ppm NO2 at 450oC. The sensor also had a short response time and recovery time (τ90) of approximately 15 and 39 s, respectively, for 50 ppm NO2 at 450oC.