High performance chemical sensor is highly desirable to detect traces of toxic gas molecules. Two dimensional (2D) transition metal dichalcogenides (TMDC) semiconducting materials has attracted as high performance gas sensor device applications due to unique properties such as high surface to volume ratio. Here, we describe the utilization of single-layer MoSe₂ as high-performance room temperature NH₃ gas sensors. Our single-layer MoSe₂ based gas sensor device shows comprehensible detection of NH₃ gas down to 50 ppm. We also confirmed gas sensing measurement by recording the Raman spectra before and after exposing the device to NH₃ gas, which subsequently shows the shift due to charger transfer and analyte gas molecule adsorption on surface of single-layer MoSe₂ nanosheet. Our investigations show the potential use of single-layer and few layer thick MoSe₂ and other TMDC as high-performance gas sensors.