In this study we have studied the adsorption and dissociation of H₂Se gas on pristine and transitions metal (TM) atoms (Fe, Mn) doped Boron nitride nanosheets. The adsorption and dissociation of H₂Se gas have investigated theoretically using density functional theory (DFT) calculations. To understand the adsorption mechanics, we have examined the adsorption energy, the charge transfer between the adsorbent and adsorbate, the band structure, the DOS as well as the optical properties. The structural stability of transitions metal atoms (Fe, Mn) doped Boron nitride nanosheets have been verified by finding the cohesive energy. Our study displayed that the adsorption energy increases in a significant range of doped sheets as compared with the pristine BN sheets. The adsorption energies of H₂Se on pristine BN, Fe-BN, and Mn-BN sheets are -0.012 eV, -7.627eV, and -10.001eV, respectively i.e. the H₂Se gas get dissociated when interacted with the Fe-BN, and Mn-BN nanosheets. The relaxed geometrical structures of complexes and electron density difference (EDD) map analysis displayed that the H₂Se gas makes bond with TM doped nanosheets i.e. dissociated. Furthermore, we viewed the optical properties of the pure, TM doped nanosheets as well as gas adsorbed complex structure to demonstrate the adsorption behavior. Therefore, our obtained results demonstrated that the Fe and Mn doped BN sheets are good candidates for adsorption and dissociation of H₂Se gas.