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For gas sensing applications, most of the reported two-dimensional (2D) materials are suffering from relatively low sensitivity and high limit of detection (LOD) at room temperature. In this work, we selected rhenium disulfide (ReS 2 ) nanosheets to fabricate ReS 2 transistor-based gas sensors (RTGSs) with ultrahigh sensitivity and low LOD toward nitrogen dioxide (NO 2 ). The ReS 2 nanosheets with different thicknesses were prepared via mechanical exfoliation and all-dry transfer method. Under 405 nm light illumination at room temperature (25 °C), the fabricated gas sensors showed a significant enhancement of the response with full reversibility toward ppb level NO 2 (response of 9.07 at 500 ppb, a LOD of 50 ppb). In particular, the total response and recovery time of the RTGS was revealed to be less than 4 minutes (55 and 180 s, respectively), which is one of the top three shortest response and recovery times toward ppb level NO 2 of the reported 2D material-based room-temperature gas sensors so far. Via Raman spectrometry, Kelvin probe force microscopy (KPFM), and X-ray photoelectron spectrometry (XPS), the structure and gas sensing mechanism of the materials were systematically investigated. It was confirmed that the electrons transfer from the ReS 2 surface to NO 2 molecules, inducing the hole doping of ReS 2 , which consequently increased the sensor resistance. Moreover, the concentration of the photogenerated carriers in ReS 2 would accordingly be promoted by light illumination, which accounts for the substantial light enhancement of the gas sensing performance of RTGSs.