To improve the NO-sensing performance of single-walled carbon nanotube (SWCNT)-based sensors, zinc oxide (ZnO) nanoclusters (NCs) were functionalized by a microwave (MW)-assisted synthesis technique. Gas sensors based on pristine SWCNTs and ZnO NC-SWCNT composites synthesized using different weight ratios (ZnO/SWCNTs = 0.5:1, 1:1, 2:1, and 3:1) were fabricated, and their ability to sense various gases at room temperature (25 °C) was investigated. The results showed that the sensing performance of the ZnO NC-SWCNT composite synthesized with a weight ratio of 1:1 (denoted as Z-SWCNTs) was significantly enhanced with respect to NO response and selectivity. This enhanced sensing performance is thought to be a result of both the modulation of the conduction channel at the ZnO NC-SWCNT heterointerfaces and the generation of defects (or holes) by MW irradiation that act as active sites for the target gases. The results obtained in this work provide not only a facile method of cofunctionalizing oxide NCs and defects but also a new methodology for improving the sensing capabilities of SWCNT-based gas sensors.