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Many toxic gases are mixed into the atmosphere because of increased air pollution. An efficient gas sensor is required to detect these poisonous gases with its ultrasensitive ability. We employed the thermal evaporation method to deposit an n-SnSe2/p-SnO/n-SnSe heterojunction and observed a temperature-dependent n–p–n switching NO2gas sensor with high selectivity working at room temperature (RT). The structural and morphological properties of the material were studied using the characterization techniques such as XRD, SEM, Raman spectroscopy, XPS, and HRTEM, respectively. At RT, the device response was 256% for 5 ppm NO2. The response/recovery times were 34 s/272 s, respectively. The calculated limit of detection (LOD) was ∼115 ppb with a 38% response. The device response was better with NO2gas than with SO2, NO, H2S, CO, H2, and NH3. The mechanism of temperature-dependent n–p–n switching, fast response, recovery, and selective detection of NO2at RT has been discussed on the basis of physisorption and charge transfer. Thus, this work will add a new dimension to 2D materials as selective gas detectors at room temperature.