The exceptional characteristics of transition metal dichalcogenides (TMDs) with two dimensions (2D) MoSe2 monolayer inspired this research to investigate different characteristics of MoSe2 nanosheet. Also, N-doped MoSe 2 nanosheet has been designed for density functional theory (DFT) calculation. According to the cohesive energy, the N-doped MoSe2 nanosheet is more stable compared to pristine MoSe2. The variation in structural, electronic, and optical properties of MoSe 2 due to N-doping has been studied along with the gas sensing ability of the designed nanosheets towards PH3, C2N2, and HN3 toxic gases. The adsorption of those selective gases on the surface of MoSe2 and N-doped MoSe2 are studied where N-doped MoSe2 is more sensitive toward the selected gases than pristine MoSe2 nanosheet. HN3 gas in N-MoSe2

nanosheet shows the highest adsorption energy which value is -0.198 eV. The

semiconductors, pristine MoSe 2 and N-doped MoSe 2 nanosheets possessed direct band gaps of 1.48 eV and 1.09 eV respectively. After adsorption of C2N2 and HN3 toxic gases in N-doped MoSe2 nanosheet, the band gaps are zero. The optical properties are very similar for both the pristine and doped MoSe2 nanosheets. All the structures show a very high absorption coefficient of 10^5 cm^-1 order which shows slight peak intensity variation in the same wavelength region due to the interaction with selective toxic gases. This work illustrates that N-MoSe2 nanosheet may be considered for sensitive detection of those selective toxic gases than MoSe2 nanosheet.